I do however much I feel like. Sometimes I do like half an hour up to 2 hours of learning. It really depends on how much time you can put in, and how much time you want to put in on learning Japanese. Just remember to have Fun learning!(or else it might feel like a chore, in stead of something fun and exciting)
As much as you can reliably without feeling overwhelmed.
Which means: If you know your live forces you to restrict your learning to say one hour because your boss gives you overtime or your teacher gives you homework or your girlfriend/boyfriend needs attention, then best plan around this hour.
Also: At some point you will be fatigued, not making any more progress on this day. This will give you the feeling of being overwhelmed. If you reach this point after x hours/minutes, then plan around this timespan. Being able to finish your schedule gives you a sense of accomplishment, whereas trying more than you can actually do will pull you down.
Finally: I try to do several sessions spread over the day. Which is possible for me, but not necessarily for you. And I feel I can do more this way than if I tried to do this all in one go.
