Requests can be made at the time of reservation (that’s when I make mine), or anytime after that. Rooms are assigned a few days before your arrival date, so to be on the safe side, I’d try to get them in no later than 5 days before your arrival.
Have tried various times at making our requests, last trip made the request about a month or so before arrival and got exactly what we wanted! Going to stick with that strategy and hope we are more good than lucky!
I make requests as soon as possible - either when booking or shortly afterwards. I also get touring plans to send in room requests for me. They email Disney about a month prior to arrival, so calling reservations about a month in advance would seem like a good strategy also.