Normally, I just stick a warm keg in the kegerator hook up the CO2 at serving pressure and let it sit. In 5 to 7 days it is fully carbonated. I do take samples from time to time to check the level.
When I am in a hurry, I hook up CO2 to the warm keg at 30 psi until the regulator stops hissing. I roll the keg around a while and hit it again with more gas. When the keg pressure hits about 25 psi according to my keg pressure guage
after rolling the keg around, I then stick it in the kegerator and hook up CO2 at serving pressure. By the time the keg cools to serving temperature, the pressure has also pretty much settled out at the right pressure. It is reasonable close after 24 hours and dead on at 48 hours. If you are starting with a cold keg, then do the 30 psi and roll routine until the pressure guage hits about 18 psi.
If you don't have a way to measure the pressure, then do the warm keg routine until the keg doesn't seem to take much additional CO2 after rolling. With a cold keg, all bets are off. This is also much simpler if you have more than one CO2 tank and regulator.
Hope this is somewhat helpful. If not, you didn't hear it from me.
Wayne
Bugeater Brewing Company