There comes a point in your patter where the audience is cued to expect music next. If you go on talking or tuning or mucking about after this moment (as bands often do), they become confused and frustrated.

Interesting thought. I'd like to say my intros carry on till they reach that point, but I don't always get that far before I dry up.

Then there are the acts who know perfectly well they're dragging it out too long, & effectively try the audience's patience for comic effect - doesn't always work, but it's very funny when it does.

I guess there are as many different kinds of introduction as there are kinds of act. Since Billy Connolly and Jasper Carrott got started there have always been people honing their patter in the hope that they'll get asked to do more of it - fair enough if you can do it, but some people would be better off working on the songs. (There's a guy I know who does what ought to be very funny songs, each of which he gives a long intro which includes - and explains - all the jokes in the song; it's a bit counter-productive.) Then there are people who just want to say where they got the song or when they wrote it, and people who just want to raise a quick laugh so that they can feel the audience is on their side. And people who just want to sing.