It really depends on the show, venue, if there is a M&G after the show, etc.
Park West is the easiest venue I've been to to meet V, as the bus had to park out on the side street (North Orleans St.) and we just waited over there for her to come out. As others have said, it can take anywhere from 15 minutes post-show to an hour or hour and a half after.
V is pretty good about coming out, but there are times if she is sick, weather is bad, they have to roll to the next city, etc. she might not.
Before the show is really hit or miss depending on the venue.