You want to clean it out just before it gets nasty, which depends on your circumstances - does your floor get damp, do you have lots of airflow, would you rather rake it out weekly or shovel out vast quantities of litter twice a year? (From experience, I'll tell you, the floor has to stay dry to use deep litter!)
People also set up things like poop boards under the roosts, which they can scrape into a bucket. This cuts down on the floor cleaning.