If we're talking about mastering, then it depends. If you have fast attack and release settings, then you can probably only get 1-2 db of gain reduction before the distortion becomes pretty noticeable, which is a problem unless that happens to be the sound you're after (ie dubstep). BUT, you can totally get more gain reduction without distorting by slowing down the release time on the limiter. I regularly take 7 db or more off my masters, I just slow things down so they don't distort. You can also run multiple limiters in series each taking 1-2 db of gain reduction if you want to keep the release fast.