How long until slide-frame become loose?

How many rounds does it take to loosen up a 1911 slide to frame fit? It sounds like a good barrel lasts many tens of thousands of rounds without loosening up too much, but how often does one have to send in a 1911 to a gunsmith to have the slide to frame tightened? If a gunsmith tightens up to slide to frame fit does it last as long as a new gun?

Depends heavily on the quality of the steels and heat treatment of the slide and frame.  I have a gun built on an old Colt Series 70 that has had many thousands of rounds and is holding up well.  I also built an experimental project gun on a Citadel (Rock Island) slide and frame and it loosened up quickly.  Can also depend on the technique used to fit slide and frame initially.

Wouldn't worry too much about it, even if the slide to frame fit is a little loose by doing a hard-fit barrel it will be tight as a drum in lockup where it counts.