Depending upon how far in advance the invitation has been issued, I either respond purt darned quick or within, say, 72 hours (for events that are more than 10 days away).
I know what you mean about the lack of R.S.V.P. skills! I used to entertain and organize events a lot more than I do nowadays. It's not so much that people are slow to respond to invites as it is that they take them sooooo casually, renegging at the last minute because "they just don't feel like going out tonight" or "got another offer."
My experience is that there's an age aspect to the attitude. I find that people in my age group (early 50s) and younger often treat invitations to an event as "optional" -- to be taken up if the mood is right, or casually dismissed if "something better comes along." In contrast, my older friends are sticklers for RSVPing and following through.
I couldn't imagine calling someone up a day or week after blowing off a party with the excuse that I just didn't feel like making the effort to show up! Or that I got a better offer! And yet I've heard those excuses offered up without the least bit of self-consciousness. Yikes! If someone does that twice to me, I cross them off my list and don't worry about 'em. We all have times when we don't "feel" like doing something, but if you've given your word, you go anyway, because someone else is counting on you.
Something else I've noticed that also seems to be age-related, at least to a degree: thank you's. When I have a mixed age group over to my house for a party, I receive simple, quick thank-you's via e-mail or phone within 48 hours from EVERYONE
over 50 and NO ONE
under 50. And we're talking about folks who are professionals 9 to 5, who ought to have a clue about basic social graces.
Maybe I'm old-fashioned, but I think offering a simple "thank you" never goes out of style!