Everyone is different but plan what you actually are going to do in each session. For me it worked better to have it so I didn't just write 'maths', but instead wrote 'maths past paper 2012' or something. You might want it to be as constricting as possible and to completely reduce your freedoms or it might be better for you to have loads if freedom if you can trust yourself. For me, breaks only served to interrupt my flow because I have a great attention span, so I'd work for 4 hours, 1 hour break, work for 5 hours, 1 hour break work for 2 hours, 3 hours off, bed. This was much better for me than regular 5 minute breaks or whatever.