I usually have a scope of some sort every year, like a sigmoidoscopy... I had a colonoscopy in July of this year and the one before that was three years ago. But they say that after you've had UC for 10 years you should get one done yearly to check for dysplasia and stuff.
As for scheduling, the farthest out I've scheduled one is like two weeks, but some doctors like to have stuff in way in advance.