The Rebbe's guidance on fixed times for Torah study and the daily shiurim (Chitas):
Set fixed times — especially when busy. Establish fixed times for Torah study, in both the revealed Torah and Chassidus, even when life feels overwhelmingly busy (IK 1057, 1283, 4435, 6249). Fixed study is a vessel for G-d's blessings, including livelihood and health (IK 4340, 4452, 5820, 6135). Keep your daily commitments even when time is short (IK 5052), use spare moments at work for learning (IK 3819), and give extra time on Shabbos (IK 9105). Even during illness, keep learning, at least a little (IK 5487).
Chitas every day. The daily portions of Chumash with Rashi, Tehillim and Tanya are for every Jew, and each person must learn them himself — relying on others doesn't suffice (IK 2740). Chumash is learned during the day, and what was missed can be made up at night (IK 1010, 8485). Keep them on Shabbos and Yom Tov as well (IK 3618). Daily Tanya is everyone's entry into the inner dimension of the Torah (IK 5712). These portions are a vessel for blessing (IK 727, 1267, 3033), and learning the same subjects as the Rebbe binds one to him (IK 649, 3620).
A balanced schedule. Follow what inspires you, but make sure regular learning includes Gemara with commentaries, halachah and Chassidus (IK 8244). A yeshiva student should follow the yeshiva's curriculum, learning both in breadth and in depth (IK 6726). An orderly daily routine with fixed study times helps both health and spiritual success (IK 6314). Learning Zohar should be regular and with understanding (IK 6310). On a birthday, add to one's learning beyond Chitas (IK 2761).
See also: Torah study – Learning Chassidus; Torah study – Diligence, motivation and memory.