Interesting how every good writing session I have lasts about 4 hours. Any more than 4, and I start spinning wheels and spewing drivel which I have to completely rewrite the next day. I've noticed that lots of famous writers (whose interviews or biographies I've read) also cite 4 hours as their ideal amount of time to work every day (though some do up to 8-9). Roald Dahl, I believe, even said he limited himself to 2 hours.
What about you guys? Do you have the magic number that works for you?