According to Terry Foster (Pale Ale from the Classic Brewing Series) for 1.0, 1.5, 2.0, 2.5, and 3.0 volumes of CO2, you need 2.5, 4.4, 6.3, 7.5, and 9.4 oz corn sugar respectively for 5 US gallons assuming the beer has 0.4 - 0.5 volumes of CO2 before priming.
Bitters usually have 1.0 - 1.5 volumes CO2 (also from Terry Foster).
Personally, I usually keg about 4 - 6 weeks before sampling. I keg after about 3 weeks in primary, and verify that the gravity has not changed for 3 days before kegging (if there's any doubt). I don't add priming sugar, but add 5 PSI to the keg, and leave it with a weekly burp on the pressure release valve. After 4 - 6 weeks, it's damned good.