You can never tell. Cancellations and re-scheduling do happen. You just never know. The most likely time might be right before the 31 day cut-off for people to cancel without the points going into holding. It will also depend on how many people are ahead of you in the waitlist.
It's good that you have a back-up ressie just in case. In August (within 30 days), you'll be automatically taken off the waitlist and will need to call to request leaving the Grand Villa wait-list in place. Last minute cancellations are possible, just don't count on it too much.
Bear in mind that if the waitlist does come through at the last minute, you probably will have some points go into holding which will need to be used before the end of your use year and can only be used for ressies less than 60 days out.
Good luck.