AC reward flight open up 356 days in advance. Other airlines are 330 (UA, IIRC). Others are somewhere in between.

If you want to be absolutely sure, you can pick an arbitrary return flight. When the timing of your actual return flight is right, you can call in and change the ticket - with a $90 change fee, of course.

Of course, it all depends on where you're going. Some destinations pretty much need to be booked as soon as they're open while others may not be as in demand.

From personal experience of booking a Biz reward trip for next year, I found even previously "hot" destinations have pretty good availabability.