I've always done a 3 month window. For one thing, my guests seem to book a month or 3 weeks in advance, not much more than that. And I never wanted to be chained to having to be around for some booking 6 months or more down the road, in case I decided to go somewhere myself.
It also seems like more possibility of guests cancelling if they book way far ahead, because so many unforeseen things can happen in peoples' lives that change their plans.
I can see where it would make sense for hosts to have their calendars open a year out if they live somewhere there is a major event that guests tend to plan and book for far ahead so they are assured of finding accomodation at a reasonable price.