I assume we mean fap or school alley rape? Somewhere around the 2-4 minute mark, towards the lower end if I'm skiving at work, or she starts sobbing.
I used to take a long time about it, until I determined that I could recover and go again up to about three times in a row to get a much higher high.
If we're talking about disgusting stuff like lights-out/under-covers/missionary, then quite a bit longer, as the sensation is significantly less intense, and pregnancy fear is always lurking.
Daily IM Poll Girl is looking very fappable again today, fap fap fap.