There are so many things that come into play, like what kinda fish you are keeping, SW or FW, and what kind of filtration your useing. 10x turn over in a wet/dry sump with a fuge may be rediculous overkill or even unpractical depending on design, while 10x HOB turn over on the same exact tank may be just right. Its all subjective and anything is possiable. Would you filter a heavily stocked 180g planted discus tank the same as a well stocked predator tank? probably not. But for the sake of this discussion, the "rule of thumb" that I have seen most often for SA/CA tanks is that you should aim for approx. 5x turn over for your average wet/dry sump, 7x for canisters and 10x+ for HOB filters.