Once you cross the point where you can kill them more efficiently with a pistol than a shotgun, that's when you swap. Personally, I've killed them with just a pistol and no pistol traits, but then it's probably more practical to just corner-kite them with a shotgun or pistol, whatever's to your taste.
In my standard progression path, I ditch my shotgun the moment I set up into my main build. For example, in a Ammochain game I stop using my shotgun the moment I get to a chaingun. I'll keep it around as a backup, with one set of shells, until I get my first plasgun. In a Pistol game I ditch my shotgun the moment I get SoG2, which is pretty quick considering that I play on N! almost exclusively.
If you're not completely comfortable with a pistol though, I'd keep your shotty around to deal with high-threat enemies like Pain Elementals. In a Ao100 Pistols game, your main threats are going to be Revs and Pain/Agony Elementals, and if you get a Cyberdemon level it's going to be brutal. Besides boss enemies and revs/pains/agonies, you can trash anything else the game throws at you. Once you have your full pistol build rolling, your time-to-kill should be low enough that you can take out anything shy of a HK in less than .3 sec, and nothing except for revs and pain elementals should take longer than .7 sec, excluding N! enemies of course.