Like Mikael says, it depends on what works for you and the strop you are using.
If you haven't stropped before you need to establish a bench mark. On a plain leather strop try 50 - 60 laps (a lap is one stroke up, one stroke down). If that's good enough, try less until it isn't so good. If it's not good enough, try more.
You can't really overstrop, but you can damage the razor by getting sloppy and making mistakes by stropping for an extended time. Personally, I find that after a point it is a case of diminishing returns, and that point for me is 30 linen and 50 leather.
After some time you will realise that its a subtle interplay involving many factors, such as how taut you pull the strop (try to just pull it firmly, don't use it like a bull-worker), letting it go a little slacker for a smiling blade, how fast you go (you can go too slowly - try for a nice even speed, not the mad barber style of slapping it up and down as fast as possible), whether the blade is fresh off the hone or not, and a host of other things.
Regards,
Neil