How long?
In many sex stories, the poor captive/happy slave has to wear a gag for days on end. In real life - well, it's highly personal but most people starting out will feel rather uncomfortable after an hour, if not before. Your mouth gets very dry, with a ball or ring gag your lips will get drooly which does not feel that great all the time (though it'll arouse your Dom). The longest I've worn a gag in one go is, like, two hours and a half, and I don't think many would ever endure to spend a whole day gagged.

A good way to prevent drooling from a ring gag is to stuff something inside (panties or socks) though that takes away the option of combining it with a blowjob...