While on this subject, how do you tell when a main or jib is blow out? Exactly how much of a difference do new sails make?
On the 18, the jib will flutter in a moderate to strong wind. You can also visibly see that it's gotten baggier when compared to a new jib and the draft starts to move back. On the mainsail, the luff will stretch and the tack will get lower and lower (i.e., less downhaul range).
Having new sails is really only important if you're racing. My 20 year old beach boat sails still work just fine for blasting around. As long as the fittings are still in good shape and the sails aren't worn to the point of tearing or being close to tearing, they're probably fine (the dacron fabric will lose its crispness).
For racing, it depends on how competetive you are and how often you race, but about 4 years would be the max for a jib, maybe a little more for the main. If you race a lot, it could be half that.