Pensum's post is a good one...I second it.
However, that schedule relates more to Creation and Completion, deity yoga practices.
I think you are talking about shinay or Lhatong practice....or a less elaborated meditation practice. In that case, it's good to have many shorter sessions within a long session. For 5 or 10 minutes, one maintains the practice...then one rests for a few minutes, perhaps reciting prayers or something, or mantra....and you can alternate like this, so in two hours you have many short "sessions" while seated.
Again, it depends on what you're focusing on.....if it's deity yoga, and creation stage focus, then one session of two to three hours means you recite sadhana up to the recitation section, then do mantra, etc., for the bulk of that time, doing the completion at the end. If it's a completion stage practice, one does the sadhana and a short recitation of mantra, followed by the bulk of the session being the completion stage. If it's some sort of silent meditation practice, like the stages of Mahamudra, or such, then you break up the long session into smaller bits.
You mentioned doing "real meditation"--all of this, prayers, guru yoga, visualizations, etc., IS real meditation. It is all the mind, right?
Based on your concerns, here's how I structure a retreat in general, incorporating the elements you've mentioned:
Morning session--pre-dawn: yoga of arising, blessing speech, transforming mantras, Zungs, and ngondro- (the four ordinary, and four extraordinary, practices--focusing on whatever you're accumulating, if you are) -also, if one takes vows, this is the time to do that.....then one begins the main practice of the retreat, until after the sun is up. You can do White Tara, then, and/or Vajrasattva, and offer Water Tormas if you do this practice. I usually recite the 21 Tara praises in the short form at the end of the first session as well, if I'm doing a solo retreat, and also I do Riwo Sangcho. Or you can do these last two at the beginning of the next session. In any case, I always seal each session with dedication prayer, usually a short one for the first three sessions, and a longer one at the end of the fourth session.
Mid Morning session: maybe Tara and Riwo Sangcho, if not done before....then the main practice. Then, before the noon break, if you have recitation commitments, you can do these at the end of this second session.
Post-Lunch session: Main practice session, and this third session is where tormas are offered, if one has these commitments to offer tormas to yidam(s). After that, Protector practices are done, including torma offerings, and I also do Sengdongma Dokpa, and other sorts of "reversal" rituals. I also do Sur Cho here....
Evening Session: Main practice session, followed by a short Kilaya practice and a short Chod practice. Then to sleep.....
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