In my experience, they will give you until 30 days out before they cancel it. Disney is VERY accommodating if you have any sort of problems. For example, on our last trip we had to leave a day later then originally planned, we called Disney that day and they moved all our reservations (2 split stays with 2 rooms, so a total of 4 rooms) so that they would begin a day later. THEN it took us an extra day to get down there, so when we arrived they offered to again move our reservations back a day, so we did not miss any Disney time. One of the rooms had actually automatically cancelled out because of us being over 24 hours late (the other did not for some reason), but they set everything back up. They did not have to do any of that, especially twice. So, just call them, let them know and they will probably just hold your reservation open for another day.
Good luck and have fun! I wish we were only 46 days out.