I assume it depends on who you talk to, but I doubt it would be very far in advance, if at all. I know I have tried a few times and never been able to book more than 30 days in advance though. Good thing is though, that they will tell you what day you can call back on.
It's really only the seasonal tours you need to book that far in advance. The Holiday Time at Disneyland tour and the Halloween tours fill up fast. You have the option to reserve your tickets that far in advance, I would suggest doing so if you're making a tour a key point in your visit, but in most cases you can pick them up days before, if not day of.