I guess we all agree that it depends on the A/C ... and of course, the pilot (won't distinguish between guts and madness for this here). For an old rivet tin can of 172 my personal "comfort zone" is all above 1.000ft long and 30ft wide, which is the size of a friends gras airstrip, where we frequently barbecue. Shortest strip I departed was a tit over 600ft long, but you have to have perfect conditions for such. 500m is plenty of space for most GA machines.