My pipeline is such that i dont get around to my beers until they are at least a month old and at least 2-3 weeks cold conditioned. Thus i dont like to wait any longer and have perfected the shake the keg method. I hook my co2 line to the beer out side of the keg, it makes little bubbles at the bottom of the keg and helps alot with co2 absorption. I set my reg to 35-45 psi and rock it back and forth until i feel im pretty close to where i wanna be, always erring on the side of caution. Iv never rly timed how long this takes, and it really depends on the headspace inside the keg, less headspace takes longer to get the co2 into solution. But heres the trick, i turn the tank valve off and continue shaking. Once the reg has drained of pressure, the low side gauge will start going down as well, keep shaking until it stops going down and thats how much co2 you have in solution. I go until i get between 15 - 20. If you're below that, open the tank valve and shake some more. Im actually thinking of getting another pressure gauge and hooking it up to a gas in disconnect so all i have to do is close the regulator valve.
Haters of the shake the keg method will hate, but iv been doing it this way for 3 years and i have only once over carbed a beer and it wasnt a big deal.