Depends what kind of a writer you are. If you write for a living (i.e. author) then the King says that you should get into a habit of writing 1000+ words a day - even if only to get into the habit of writing every day. Granted, it may be pure crap, but you may still wind up with something useable.
(By 'the King' I mean Stephen King. Check out his book 'On Writing', it has a few answers to your questions)
As for me, i get ideas and scenarios in my head at the most inopportune of times so i've taken to carrying around a pen and small notepad with me so that I can get the actual idea itself down. The finer details can be worked out later when i'm at the computer, and actually writing. And as far as drawing is concerned, I put on some music and start straight away, even if i'm not in the mood. If I don't, I could wait from now til the end of time to 'get in the mood', but if I actually start, I find myself getting swept up in things, and before I know it, 4 hours have passed.