It depends on the surface area exposed (and whether you are shaking the keg).
For the short corny's I've seen, the diameter is the same as a 20L, so the you have the same surface area for half as much beer when the keg is standing. So half the time should work if you're not shaking it.
To carbonate in 10 minutes I suspect you're holding the keg sideways and shaking it. In that case the surface area to beer ratio is probably similar for both kegs, so expect the same time.
A trick I use is to dial down the regulator to the target pressure, give a shake and listen for CO2 going through the regulator. When I can't hear it I know to stop.