I was reading a great book on writing called Plot and Structure by James Scott Bell and he gave one bit of very useful advice: Don't allot a set amount of time for writing but rather set yourself a word goal for each sit down. After all, you can sit down for hours and write nothing.
Mind you, Mr. Bell had no comments on how high (or low) that limit should be. So lets say I'm working on a curve. That said, I'm starting to get a rhythm.