Is it as effective to practice with many shorter session a day as it is to practice one longer session a day
I personally prefer to practice drawing with many shorter session divided throughout the day (about 30 min to 1h) rather than drawing one longer session a day because it doesn’t feel as time consuming. but I’m not sure if it’s as effective when it comes to skill improvement. Do you think I should draw one longer session a day, divide my practice time or do whatever feels better?
Well, each study has its own virtues. Shorter ones are to make you learn about spontaneity and not being afraid to be bold with your stuff, because the study is so short that making wrong lines or values doesn't feel like you've done a crime. Meanwhile the longer ones are to make you learn about all the things that you need to make an artwork great (shape, form, values, color, lines, and all that) in thorough way. There's no one better that the other, they develop you in a different way. So I will advice you to do both studies, the short ones and the long ones.
You're doing it right. Work for a focused period, then get up and take a break. Sometimes you'll get in a flow state and want to work longer, and that's fine. Just make sure you are doing longer studies and not only 30 minute studies. As long as you are practicing doing finished work as well as short studies, I think it's good. We have this weird idea in modern day culture of the 9-5 job, where you work 4 hour chunks. But how much of that time is checking email, zoning out, chatting with friends, and generally bullshitting? Better to do many highly focused intervals than one sloppy 3 hour chunk