Here's my little secret:
I have sankey kegs and I am able to carb in 24 hour consistently. I switch the lines on the coupling head so that the CO2 is going in the beer out line and I put a coin (dime) in the beer line between the gasket and tail piece and connect it to the gas-in line.
With that setting crank up the CO2 to 20psi and let it sit over night. In the morning give the keg a bit of a shake, nothing major but about 30 seconds of agitation. Let it set until the 24 hour mark has passed. At that point in time, turn off the gas and purge the keg. Reverse the lines and restart the gas at serving psi (10psi for me) and you should be ready to go.
Works like a charm!