If you dedicate 10-15 minutes every day… and really commit to it , it’ll become a long term habit ! Whereas if you only do longer periods of time once a week you are more likely to fall out of the habit! Even if it’s just 5 Mins a day, do it consistently every day and you will see results!!
I do prefer to study for long periods of time, but since I'm working now, I have to adapt my schedule to add studying in it. For that reason, I believe it's healthier for me to "eat it in pieces", which means to learn a little bit every day. I guess it's just a matter of what fits in your agenda.
Personally I prefer an environment with a few hours per day (2-4). But due to my lifestyle (work/uni) it usually ends up a longer block of learning every few days, where I find myself completely switched off on the off days, and it's hard to get back into a routine/get on a roll on my learning days.
I'd say both. It's hard to predict which day at which time I shall have the optimum mood, concentration, energy, willingness, the perfect time and environment. So, I'd begin with learning without planning which parts to learn which day. I'd simply begin to learn and concentrate on what I'm learning and I'd continue that (could be a few hours) as long as all the factors remain optimum.