uhhh, how much CO2

OK, I read alot in the stickies and around here and there, and I still am a little unclear on this: I want to force carb, so I set my reg to 30 psi and shake with the gas going. I did that for say five minutes, then put it in the fridge to chill. This is a Belgian Strong Ale. I assume once its chilled I do this again. Is it just trial and error to get the right amount in? How many times do I do this?
Thanks guys...

The best, most consistent method of force carbonating is the "set it and forget it" method. Just find one of the many carbonation charts on the web, find the proper CO2 pressure, based on the style of beer and the temperature that the keg will be at, set your regulator to that pressure and forget about it for a few days. No guess work, and perfect, repeatable results every time.

Last weekend i built a bleeder valve i have only used it once so far but it is possibly the most awesome gizmo in my homebrew arsenal. and not to mention i can have beer carbonated in under 24 hours and actually know what pressure my keg is it. So basically i achieve the same goals as the set it and forget it method, but in 5 minutes.

Thanks John and Marty