[Pinski]

... before you kick it down to 8-10 psi for service? 

[liquidbrewing]

If your beer is good and cold, crank it to 30, shake the crap out of it for a mintue or so.  Leave it at 30 for 2 days total, then vent the keg and set it at your desired serving pressure.  (8-10)  You should have good carbonation in 5-7 days total, just test it on the fifth day.  That's my force carbing schedule anyways.  It's worked great for me.

[tumarkin]

depends on the style you're brewing (hefeweizens are more highly carbonated than english bitters, for example) and also on temperature. the colder the beer is, the more easily/quickly it will accept carbonation. refer to one of the many online carbonation charts, such as http://www.kegkits.com/carbonation.htm for guidance on the amount of pressure to set your regulator at a chosen temperature to achieve a specific carbonation level. it's not a quick process, but you can speed it up by shaking the keg, and accelerating the rate at which the CO2 enters the beer.

[oscarvan]

There's many ways......

If I'm in a hurry I crank it up to 30 and sit there a rockabyebaby the thing for 10 minutes..... voila....drinkable, though not perfect. Service pressure for a few more days and it's good.

[bluesman]

I force carb at 30psi for 48hrs then reduce to serving pressure and hold there. I don't use the "shake method" unless I'm in real hurry which is very rare because I typically have several beers on tap at any given time. I also use the "set it and forget it" method if I'm not ready to serve the beer really soon.

[narvin]

It depends... there's no true way to estimate. That's why I always set at serving pressure... you can still shake to speed up carbonation.