Gemara

(Mishnah): Reuven may be Sho'el (borrow) barrels of wine or oil from his friend, as long as he does not say 'Halveni' (lend to me);

Question (Rava bar Rav Chanan): What is the difference whether he says 'Hash'ileni' or 'Halveni'?

Answer (Abaye): If he says 'Halveni', the lender is prone to write [details of the loan]. If he says 'Hash'ileni', we are not concerned lest he write.

Makos 3b (Beraisa): If one lends without setting a date, he may not demand payment until 30 days.

Kesuvos 34b (Rava): If a man died and left a borrowed cow to his sons, they may use it as long as he was allowed to use it. If it dies, they are exempt.

Tosefta (Bava Metzi'a 8:11): If one is Sho'el an inn to lodge in, this is for at least one day. One who borrows a garment to go to a mourner's house, it is in order to go and return. If he borrowed to go to a nuptial feast, it is for the entire day. If he borrowed for his own Avelus or Nisu'in, it is for at least seven days;

Tosefta (10:1): One who is Malveh Stam, it is not for less than 30 days. In a place where the custom is to lend for less or more than this, we do not deviate from the custom.

Rishonim

Rambam (Hilchos She'elah 1:5) If Shimon borrows a Kli or animal from Reuven, he returns (takes it back) whenever he wants. If he borrowed for a set time, the owner may not take it back until the end of the time. Even if Shimon died, his heirs use it until the end of the time. A buyer acquires the object itself permanently through the money he pays. One who receives a gift acquires it permanently without paying anything. A renter acquires usage of the object until the set time through the money he pays. A borrower acquires usage until the set time without paying anything. Just like a giver is like a seller, i.e. he can never retract, a lender is like an owner who rents out, and he cannot retract within the set time.

Magid Mishneh: Makos 3b says that one cannot claim a Stam loan within 30 days only regarding a Halva'ah, which is given for Hotza'ah (the borrower spends it. He does not return the same item he borrowed.) Rashi explained like this in Shabbos (148a DH Gemara), and also the Ramban and Rashba. The Tosefta proves like this, unlike some Meforshim who say that a Stam She'elah is 30 days, just like a Stam Halva'ah. The Rambam's opinion is primary.

Rosh (Makos 1:4): R. Tam (Sefer ha'Yashar 483) says that just like a Stam Halva'ah is for 30 days, also a She'elah. A support is that a borrowed Talis is exempt from Tzitzis for 30 days. This is no proof. Rather, it is rare to lend for more than 30 days. We obligate it, lest people think that he bought it. The Halachah is, a lender can retract whenever he wants. A proof is from Shabbos 148a. We are concerned lest one write if he says 'Halveni', for normally it is for a long time; a Stam Halva'ah is for 30 days. We are concerned when he says 'Hash'ileni', for normally it is for a short time. Maseches Semachos says that if one borrowed a garment to go to a mourner's house, festival or nuptial feast, the lender cannot take it back until the end of the Avelus, festival or week of Sheva Berachos. This implies that normally, he could retract before this.

Note: Also other Rishonim cite this. It is not in our Maseches Semachos, but a Tosefta (Bava Metzi'a 8:11) says like this.

Ran (63a DH Gemara): Rashi (148a) explains that when he says 'Halveni', the lender is prone to write, for a Stam Halva'ah is 30 days. This implies that She'elah has no fixed time. The lender may take back his item whenever he wants. Therefore, one must say Hash'ileni. Even though he does not return the same item he borrowed, and it is proper to call this Halva'ah, and not She'elah, he means 'I will return it to you whenever you want, like a She'elah.'

Nimukei Yosef (Bava Metzi'a 57a DH Kach): Meshichah was enacted also for Shomrim, i.e. after Meshichah the lender cannot retract, even before the borrower used it. Just like Meshichah prevents one who rents out his item from retracting, it prevents the renter from retracting. It is unreasonable to say so for a borrower. Granted, Meshichah prevents the lender from retracting, but the borrower can retract. The loan was for his needs. We say "he gets all the benefit." The Meforshim say so. The Rosh says that Chachamim made a uniform law, and also a borrower cannot retract. Surely, he may return to the owner's Reshus. He need not guard against his will. However, if Ones occurred while returning it, we do not say that once he wanted to return it, it is as if the She'elah ended, and he is [exempt] like a paid Shomer.

R. Akiva Eiger (CM 341:1): See 293:2. (If one gave a deposit in a Yishuv (settled place), he need not accept it back in a Midbar. He may say 'you are responsible for it until you return it to me in a Yishuv, like I deposited.)

Rashi (34b DH Kol and DH Ein): The heirs of the borrower may use the item for the period for which it was lent to their father. [After this] they are exempt if it died, for they are not borrowers.

Shitah Mekubetzes (34b DH v'Li): I say that even if Stam She'elah is 30 days, this is only for the borrower. If he died, his heirs may use it only until the time their father specified. Rashi was precise to teach that this is why the Gemara taught this law (an animal that died after the period for which it was borrowed) regarding heirs, and not regarding a regular borrower. (A regular borrower would be liable - PF.)

Avnei Milu'im (28:50): The Rosh (Kidushin 1:20) taught when Kidushin with a borrowed item works. One condition is that he borrowed it for a fixed time, for if not, the lender can demand it back immediately. Also, he must ask permission to lend it to another. The Terumas ha'Deshen (210) says that without permission, one may not lend to another, but if he does, it is not Gezel (theft). I say that if he did not lend for a fixed time, even those who say that it is for 30 days, lending to another is Gezel. The Shitah says that even if Stam She'elah is 30 days, this is only for the borrower, but not for his children. All the more so it is not so for a third party to whom the borrower lends.

Poskim

Shulchan Aruch (CM 341:1): If one borrowed a Kli or animal Stam, the lender can demand it back whenever he wants. If he borrowed it for a fixed time, once he did Meshichah, he acquired, and the owner cannot take it back from him until the time ends. Even if the borrowed died, his heirs use it until the end of the time.

SMA (1): This is unlike R. Tam, who equates loans of items and of money.

Be'er ha'Golah (1): Rashi, the Ramban and Rashba hold like the Magid Mishneh. The Tosefta proves like this. This is primary.

Pischei Teshuvah (1): The Urim v'Tumim (73, Urim 4) wrote that if one borrowed for a specific need, e.g. an ax to chop with, one must let him chop at least once. For that matter, it is as if he lent it to him for a set time.

Nesivos ha'Mishpat (Bi'urim 1): Even according to R. Tam, who holds that Stam She'elah is 30 days, after 30 days it is as if the time of the She'elah ended and he is exempt from Ones. Also, the borrower may not use it any more. However, we hold that the lender can retract from a Stam She'elah at any time, so the She'elah does not end until he demands the item back. The borrower is liable for Ones, and may use it until the owner demands it.

Shulchan Aruch (OC 307:11): One who borrows from his friend on Shabbos may not say 'Halveni', which connotes a long time, so there is concern lest he write. Rather, he says "Hash'ileni." In other languages, which do not distinguish the two kinds of loans, one must say "give to me."

Avnei Milu'im (28:50): A Chacham said that in other languages, Stam She'elah is 30 days. This is wrong. Chachamim enacted 30 days only for Halva'ah, which is for Hotza'ah. Chachamim did not enact at all for She'elah, for the matter itself returns. Shabbos 148a discusses borrowing wine. One does not return the same wine. The Ran (63a) says that if he said Hash'ileni, he shows that it is not a Halva'ah which is for 30 days. Rather, it is due immediately. Other languages do not distinguish, therefore it is for 30 days, so it is forbidden on Shabbos. The Rema did not comment in CM 341 that it is different in other languages, for there we discuss a Kli or animal, and it itself returns.