Best way to see if a catch can really works is to install 2 of them. Have one dump to the other. If the 2nd one is collecting anything, then you know the first can is not working like it should....
Anyhow, with that being said, I empty mine every 1,000miles and have about an ounce of crap each time.
Also, PRIOR to every oil change (5K) I do the DI/Turbo intake clean. I made a special fitting on CAI after my MAF, spray in 1-2 second bursts at 2000rpm, let soak for an hour, run it down freeway for 10-20 miles. dump oil/filter, rinse repeat.
35K, runs like a top!