With guided meditations, I think you should try and meditate for the whole session. With unguided meditation I think meditating until you feel peaceful and at rest is best.
I try for at least 10 minutes a day! The biggest tip I have is allocating A LOT more time than you think you’ll need. Give yourself half an hour for a 10 minute meditation so you can properly relax and be present, and if you feel yourself slipping you don’t have to feel rushed to “do it right”
10 minutes is good, but if you can do 1, 5 that's better than nothing. Longer is better. 15-30 ideal. Calm app and headspace go for about 10 a day.
Personally, I use the Headspace app to meditate for about 3-5 minutes daily, but it depends on the person who is meditating. I don’t think meditating for more than, say, 7 or 8 minutes a day will fit easily into a routine(especially if you use an app) at night, so I recommend meditating in the morning. In all, it depends on your own preferences.
You should meditate at least for ten minutes every day, but if you can't do that for whatever reason, then you should meditate just for a couple minutes, to keep reinforcing the habit.