When I was freelancing, the most I scheduled was 18 months out . . . BUT, I hate the fact that I was scheduled that far out - - - I missed some wonderful opportunities of shows that popped up - and I didn't want break my word.
I think it depends on your market . . . here in the DC area . . . people hire by the season, so, people do the hiring for the entire season between Feb - April. So, you might as easily book an entire year in those months.