I am going to be force carbonating a 5 gal batch for the first time in the next day or so. Unfortunately I have left myself little time to do it - beer must be ready to serve in 3 days. A friend of mine told me he carbonates at 35-40 degrees, sets the pressure to 30psi, and leaves it for 2-3 days and it’s ready to serve. I’ve also read that I can route the CO2 in the out post and up through the bottom of the keg, set the pressure to 30psi, gently rock the keg for 15 min, then remove the gas and let sit. After an hour, connect the gas to the in post at the desired serving pressure and the beer should be ready to serve. Any thoughts on either of these methods? Appreciate the help in advance. Cheers 
My method use to be…
Rack to keg, hit the keg with 45 or so psi, and shake it like hell for 1 minute. After that 1 minute shut off the gas on the tank and shake it until the needle stops falling. You are looking for roughly 30psi at room temp. Repeat adding gas, shake, shut off gas shake, until you reach 30 psi.
If you are crash cooling, you only need about 12psi once you shut off the tank. Either method will get you carbed beer in minutes.
If you have the beer already cold 35-40F, then simply crank the regulator to 30-40psi for 2 full days, release pressure relief valve, back down to serving pressure and away you go depending on how much carbonation you desire.
Similar here, when cold (which they usually are, from cold conditioning): 40psi for 24 hours, turn to 25psi, bleed the keg then hold for 24 hours, 10psi, bleed, then serve. But many ways to get there: Sparkle & Fizz: Carbonation Methods | Brülosophy
If you only have a day the only way to do it is the roll/shake method. If you have a little more time, what I like to do is to connect the gas to the liquid out post and turn the pressure up to about 20 psi. That causes the CO2 to bubble up through the beer, carbonating faster… though not as fast as if had a diffusion stone on the end of the tube.
Also the colder the beer is, the more rapidly it will assimilate CO2.
Thanks for all the advice and suggestions. Once it was cold, I set the PSI to 40 for 24 hours, bled the keg, then 30 for another 24 hours, then bled and set to 8-10. All it was pouring was foam, so I warmed it up to room temp for 24 hours and bled the keg a few times. It’s still pouring foam. Any ideas or suggestions?
I can’t imagine that the beer would be that overcarbed at that rate. Did you chill the beer back down after allowing it to warm up to room temps? What length line are you using to pour your beers?