There really isn't a time limit to reallocate a reservation but it is up to the discresion of MS to allow you to do it. You can't reallocate to get around a home resort priority unless there is not a waitlist. For example, say you have BCV and OKW points. You make a BCV res at 11 months, you can't reallocate the reservation to use OKW points at 7 months if there is a waitlist. If there is not a waitlist, they will probably allow you to do it, it would be the same as canceling and rebooking the reservation.
I reallocated a reservation right after I bought because you can't used banked points until after closing. I made the reservation with current points and then reallocated after my closing to use the banked points.