It would depend on when you need the RSVPs by. Make sure that you give yourself a date that is actually 2-3 days before you really need them. That gives you a day to call everyone who didn't respond "to make sure they received their invitation" and a day for them to call you back and then another day to follow up with those you STILL didn't RSVP.
I'd then mail them out 2 weeks prior to that date. That gives them time to get them in the mail, check their plans, mark their calendars and get back to you. And it really should (in this theory) give them ~10 days to respond. Much longer and people will forget about it totally.