I think for most Marriotts you can book reservations around 350 days out.

Unfortunately, though, based on some recent posts in this forum, not all hotels are putting their awards inventory into the system at that time. So if you can make a normal hotel reservation for something 350 days out, but there doesn't seem to be any Standard or Anytime awards available, you might want to call the Hotel up directly and ask them about how full they are.