When you're designing your wedding invitations and RVSP cards, you have to put a date by which you expect a response. Our tip for this: your "RSVP by" date should be 8 weeks prior to your wedding date.
You'll want to at least add two weeks to the date by which your caterer needs a final meal count. If they need a final count two weeks before your wedding day, your RSVP by date should be four weeks before your wedding day.
You WILL have guests who don't respond on time. Give them a little extra leeway by not actually needing their response on the day the card says. Also, you WILL have to track some responses down and contact people. Give yourself this leeway too in order to find the time to reach out to them!