Carbonating beer can be tricky, especially on a homebrew system where you don't have access to carb testing tools. As homebrewers, we're usually pretty excited to try our new creations and letting hte beer sit under pressure for 7+ days isn't always desireable. You've likely tried (or at least seen) force carbonating a beer by agitating the keg. This method works when you're in a time crunch to get your beer carbonated, but it can be tedious and it can be very inaccurate leaving you with undercarbed or overcarbed beer.
If you're looking to get well carbonated beer in under a week, try using either of the followign options:
With the Carbonation Wand or the Quick Carbonating Lid and the proper technique (involves a little bit of babysitting), you can use to get full carbonation in about 12 hours. This involves using the pressure relief valve (PRV) to vent head pressure every couple hours. It's not difficult to do, but it does require a little scheduling to make sure you can be near your keg. Repeat this process until your keg reaches desired carbonation levels.
If you don't feel like babysitting, just connect the carb wand/quick carb lid to pressure pressure and keep the keg in a cold environment, you can achieve full carbonation in 36-48 hours. Still much better than merely using head pressure.