If you're new to drawing, or if you've never had formal life drawing classes before, start with the 30m class mode. This will run you through a perfectly serviceable 30m warm up session suitable for both new artists and working professional artists. It is a small enough chunk of time that most people can complete it and not be utterly worn out after. And it's a mode that professionals will often use as a warm up! It's something that offers a lot to artists of all levels.
As a new artist, the 30s and 1m drawings can be super duper uncomfortable. They might feel like you're not learning anything, or you're doing pointless scribbles. For the first 5-10 sessions, do your best to work past that feeling of discomfort. Sometimes we make bad drawings. Sometimes it can be really uncomfortable to make drawings that aren't finished. Or your brain may come up with other excuses for why short scribbles are bad. Brains are very good at putting the brakes on learning! Try to focus on the fun of the 5 and 10m drawings, and the way drawing after a good warmup feels better and goes more smoothly.
As you become more advanced, there may be times where it's worth doing other kinds of practice sessions. But always be super suspicious of any goal that encourages a mindset of longer classes here is better, without a concrete reason WHY you want to go longer. You want to save the long and complex drawings for your fun drawing time, not for your homework drawing time, BECAUSE drawing should be fun.
To make optimal progress in drawing, you need to focus not at the amount of time you spend, but at its quality. You need to be focused, relaxed.
In general, it is best to train for 3 hours a day (with breaks). How to devide the time depends only on you. But if you mean that it's hard for you to start from big among of time, you can start from 5 min, then instrise your practise time from session to session