For some, pumping creates more permanent results.
How much you might gain by pumping before sex depends on a lot of things, including how large you are to start with, how elastic you are, and what pumping routine you're employing. You will probably gain something. How much varies by the individual.
How long it takes before it goes back to normal also varies, but I think it is safe to say that it would be measured in hours, not minutes. The greater experience that you have with pumping, the longer the gains last, and that's how temporary gains turn into more permanent gains.