I force carbonate all my beer. Set it to 20psi and let it sit for a day or two. Bleed out the pressure until it's around 10psi, then draw off a glass. If it's carbonated, leave it at 8-10psi, or whatever pressure you want for the desired carbonation level. If not, crank it up to 20psi and give it another day. After a while you'll get a feel for how long it takes to carbonate. Also, it's not a good idea to leave the beer line on the keg at high pressure. I had a fitting come off once while it was at high pressure, and when I checked it the next day the keg was empty, the CO2 was empty, and the fridge was full of beer. No fun.