It’s okay to fall asleep. Meditation has one goal: to notice how you are right now. At least you are noticing you’re sleepy, you’re alive. There is no pass or fail for meditation-there is only showing up. An old Zen saying goes “if you don’t have time to meditate for an hour, meditate for 2 hours…” you will find your inner clock expanded by learning to free yourself of your own restrictions. Peace and blessings on your journey.
So, your first two questions will depend on you. That being said, there is always the option for your third question, you could intentionally meditate in bed right bef sleep, so if you fall asleep in the middle it won't matter. Or if you have the choice for setting your own interval bells, have a final chime that is loud enough, or set an alarm with a gentle tone a few minutes after you're supposed to finish meditating if you need to do something else afterwards.
Meditate for as long as you like. I like to do 10 minutes each time. Falling asleep is not a big deal, but be sitting – lying down is a little too chill
Try to make your meditation the same amount at the same time every day. If you fall asleep during mediation, don't worry! Mediation is a skill to be honed like any other. If you continually find yourself falling asleep, try meditating in the morning as part of your morning routine.
Meditation can be done 10-15 minutes or even 15-20 minutes but 20 minutes is long enough. I do about the same time whether its morning or night. Honestly anywhere between 10-20 minutes is fine n twice a day is usually good. If you are falling asleep try meditating sitting up instead of laying down.