I've had one on my 2010 GC for a couple of years, and depending on the mix of city/highway driving, it can fill up pretty quickly. I empty mine every 2,000 miles or so, at which point it will usually be 1/3 to 1/2 full. Highway miles seem to fill up the can more quickly- I'm guessing the higher vacuum at part throttle has something to do with that.
2010 Grand Cherokee Limited Hemi Q-D II, 4X Belly Guard. Diablosport InTune/HemiFever tune, K&N drop-in, 265/65/17 Hankook Dynapro ATM RF10's