I usually just use the chat feature. It doesn’t matter how far in advance but I have read that they start assigning rooms about 4 to 5 days ahead of time..so, I would say by then,
Since requests are not guarantee, they aren’t granted based on when they are put in.
If using your own points, call/email/chat with MS. If you booked with cash through Disney Reservation Center, call and add the request to your reservation. If you rented points, contact the owner to add the request.
I always contact Disney directly and i do it months a head of time. Also, when you check-in online do not select a room request as I heard that will trump whatever you requested by phone.