I would say at least 6-8 weeks out. I'm getting married October 2nd and my RSVP date is September 2nd. My invites would be out right now if the USPS didn't lose them and had to have my invite lady redo them. As soon as they get to me, they're going out and hopefully that won't be any later than August 2nd.
If its an out of town wedding, I would give a couple of extra weeks to give people time to make the arrangements.