It depends on how well I know them and whether they give the invite directly to myself or to the kids. I normally RSVP as soon as I can just to get it out of the way, however from previous experiences with my own kids parties, there are a mix of those that will reply early or late or not at all... My DD1's party is this Sunday and I was getting RSVP's from when I handed them out a couple of weeks ago right up until the day after the RSVP date... even now a couple of parents aren't sure if they're bring the kids or not so we plan for 22 and see how we go on the day.