This one always starts a firestorm of replies... so let see we happens...
If you are talking "return pump" than one that gives 6X-10X tank water volume is ok IMO....
Now lets say you are talking total flow, power-heads plus return, etc... (the needs vary) In a FOWLR, 10X-20X is fine... Reef tanks with mostly LPS some softies, 20X-30X... SPS dominated reef tanks, 30X-40X or higher is preferred....
As with anything in this hobby, there are also other requirements and not just the actual numbers you should be looking at. Varying directions of flow, dead spots are other important considerations. IMO I feel it's the best to use multiple power-heads to achieve the needed flow for the type of tank you're keeping, this will help you achieve better crossing flows and creating turbulence with varying degrees of flow.