as said before, depends on the situation.
if you just need a quickie to sort Jr. out and generally aleviate certain thoughts, then even a minute will do...
...but, if you really want to have one body destroyingly powerful orgasm (or your SO is being a tease) then you can go on for hours. so far my record is 7 1/2 hours, needless to say, i had a few things floating around my system, but it was damned fun none the less
