Gemara

6b (Mishnah): There is no Shi'ur for the following Mitzvos: Pe'ah,,, and Talmud Torah.

Menachos 99b - Mishnah: At the same time that Kohanim remove the old Lechem ha'Panim from the Shulchan, Kohanim opposite them put the new Lechem on, immediately filling the void, to fulfill "Lefanai Tamid";

R. Yosi says, even if the new bread is put on after the old bread was totally removed, this fulfills Tamid.

Beraisa - R. Yosi: Even if the old bread was removed in the morning and the new bread was not put on until evening, this is fine;

"Tamid" teaches that the Shulchan cannot be without bread for (even part of) the night.

R. Ami: We learn from R. Yosi that even if a person learned only one Perek each morning and evening, he fulfilled the Mitzvah "Lo Yamush Sefer ha'Torah ha'Zeh mi'Picha".

R. Yochanan: Even if a person said only Kri'as Shma morning and evening, he fulfilled Lo Yamush;

It is forbidden to tell this to an ignoramus (lest he think that his son need not learn more than this).

Rava: It is a Mitzvah to tell this to an ignoramus!

Question (Ben Dama, R. Yishmael's nephew): May someone like me, who learned the entire Torah, learn Chachmas Yevanis?

Answer (R. Yishmael): "Lo Yamush...v'Hagisa Bo Yomam va'Laylah" - if you find a time that is neither day nor night, at that time you may learn Chachmas Yevanis.

This is unlike R. Shmuel bar Nachmani.

R. Shmuel bar Nachmani: "Lo Yamush..." is not an obligation or Mitzvah. It is merely a blessing that Hash-m gave to Yehoshua, for He saw that Torah was very dear to him.

Tana d'vei R. Yishmael: Do not consider Torah to be a Chovah (that you fulfill and are then exempt from it), for you cannot exempt yourself from it.

Nedarim 8a - Rav Gidal: If one says 'I will learn this Perek', this is a great Neder (really, it is a Shevu'ah).

Question: He is already bound by an oath (i.e. of Kabalas ha'Torah) to learn. A second oath does not take effect on this!

Answer: If he wanted, he could exempt himself by saying Shma morning and evening; therefore, his oath takes effect.

Rishonim

Rambam (Hilchos Talmud Torah 1:8): Every Yisrael is obligated to learn Torah...he must fix times for Torah by day and at night - "V'Hagisa Bo Yomam va'Laylah".

Source (Chidushei R. Meir Simchah Sotah 42a DH Amar): This is from Tanchuma Beshalach (20).

Question: Regarding Lechem ha'Panim, the Rambam rules like Chachamim (that "Tamid" means without ceasing). If so, why does it suffice to fix times at day and night? This is like R. Yosi (Menachos 99b), but Chachamim require one to learn without ceasing to fulfill "Lo Yamish"!

Answer (Lechem Mishneh): "Tamid" connotes learning without ceasing more than "Lo Yamish". Therefore, R. Yosi surely does not require learning without pause. Surely Chachamim agree regarding Torah. If not, there would be no time to do one's work!

Ran (Nedarim 8a DH Ha): A person is obligated to learn day and night, according to his ability, to become able to answer questions precisely (Kidushin 30a). Kri'as Shma does not suffice for this! The Gemara means, since the Torah does not explicitly command more than Shma morning and evening, his oath takes effect totally.

Ranbi (in Shitah Mekubetzes Nedarim 8a): Saying Shma morning and evening fulfills the oath of Kabalas ha'Torah. One must engage in Torah the entire day to fulfill "V'Hagisa Bo Yomam va'Laylah".

Or Yisrael (27): "V'Hagisa Bo Yomam va'Laylah" is a Mitzvah to learn every moment. Additionally, we are commanded to know that the entire Torah. The latter Mitzvah permits activities which cause one to temporarily cease learning for the sake of greater understanding, e.g. to travel to learn from a better Rebbi, to suggest false Sevaros to sharpen Talmidim, and to rest (to enable learning deeply later, even though one could force himself to stay awake and recite verses).

Re'em (in Shitah, ibid.): Mid'Oraisa the vow to learn a Perek does not take effect because he is already obligated from "V'Limadtem Osam". The Gemara means that the vow takes effect mid'Rabanan.

Poskim

Shulchan Aruch (OC 156:1): After learning one should attend to his business. One's business should be haphazard and his Torah should be constant, then both of them will succeed.

Chachmas Shlomo (DH veha'Gaon): The Hafla'ah explains that one who is distracted with business must Kove'a i.e. 'steal' time for Torah.

Mishnah Berurah (2): Each day one should work as much as is necessary. The Yetzer ha'Ra entices people to think that they must work the entire day. Rather, one should imagine what he would consider sufficient if he had to feed and clothe another person, and do this much for himself (Sha'ar ha'Tziyon 3).

Kaf ha'Chayim (7,8): SHLaH says that one who must earn a living should spend eight hours a day working, eight hours learning and eight hours sleeping. If he need not work, he must fulfill "V'Hagisa Bo Yomam va'Laylah". In particular, one who receives stipends to learn must be very careful not to waste any time. Medrash Shmuel (1) says that one who fixes times for Torah annuls the covenant - "Es La'asos la'Shem Heferu Torasecha". Rather, one must try to learn at all times; if one finished his work early, the saved time should be for Torah.

Shulchan Aruch (YD 246:1): Every Yisrael must fix times for Torah by day and at night - "V'Hagisa Bo Yomam va'Laylah".

Rema: In pressed circumstances one fulfills this by saying Shma morning and evening.

Shach (1): R. Yochanan said that it is forbidden to tell an ignoramus that one is Yotzei through Kri'as Shema. The Rema omits this because Rava said that it is a Mitzvah to tell him. Rashi explains that the ignoramus will realize the great reward of Kri'as Shema, and all the more for learning all day! Alternatively, he will realize this by thinking about Chachamim who spend the entire day learning and do not exempt themselves through Shema.