Part of the reason is to do with the subconscious brain. Basically, are conscious brain can only access a small amount of the information we hold and poses. Yet our unconscious can access a great deal more. This means that when we practice on something we are only using our conscious minds to solve to problems we face. Maybe those problems are working on a particular lick or trying to transcribe something etc. But, what happens when we finish a practice session is that our unconscious mind keeps working on that problem. People encounter this all the time. Have you ever had to do brainstorming as part of a group at school? Did you ever notice how you hardly ever had good ideas during the brain storm but maybe an hour after it was over you had a brilliant one? Or perhaps you were able to think of the perfect thing to say in a conversation a few minutes after it finished? This is the reason why.
Regarding the amount of practice you do I would suggest doing a bit everyday rather than a huge chunk every now and then. Part of practice is also maintenance which requires a regular workout for your hands. It's like fitness in a way. Being generally fit requires some exercise everyday but if you tried to cram the equivalent of years of general fitness into one gym workout you'd never get through because you'd be exhausted.
Link Removed in which I briefly discuss something known as the "twenty minute rule" in practice. Basically, your brain can only take in new information at its optimum capacity for around twenty minutes before it needs a break. It's of course physically possible for us to go for longer but if you take breaks every twenty minutes you get a few benefits.
- You have limited time so you're way more focussed during practice
- You give your unconscious brain a chance to process everything you've just worked on
- Your muscles last for longer because they get regular intervals to flush out lactic acid
- You get a chance to recharge your body with food and water when you need to
I would suggest doing a little each day. I have found it useful to go for roughly twenty minutes practice, twenty minutes of rest and then keep alternating between the two.
Best of luck with your practice!