Depends on the girl..... Some will allow MSOG, and some won't. Would really suck to finish early and watch her start getting dressed. Discuss the issue if it's common with the dispatch and/or the girl ahead of time. Not always easy I'm sure.... But if she knows what to expect you's can make the most of it with 2 or 3 shots!
Killing some time with foreplay probably not a bad idea.... Seriously, when you go down on her, pack a lunch and stay awhile!
Cheers!