How much time is an RSVP needed before the date of the wedding?

I would suggest you RSVP as soon as you receive the invitation - if you are sure if you are attending or not. This way you won't forget to return the RSVP card. Otherwise, note the date that this card is due back (it is written inside the card when they need to have your answer) and mark it in your blackberry or on a calendar so you don't forget to send it back.