Two types of flow... the turnover in and out of the sump and the flow generated by powerheads or closed loop systems. The flow in and out of the tank should be fairly low, 3-5x max so in a 100G tank you should have between 300-500gph, basically enough to fill the tank up and spill over at a nice rate. Flow inside the tank itself, in a 100G a vortec will throw out 3000gph, so that's 30x. In some larger systems like a 300g, some of use have even more. In my 300 I have two vortecs. a polaris and a single tunze that combined put out around 9000gph, plus a wavebox that creates quite a bit of movement, though not linear movement.
I've seen 300 systems with 4 vortecs, so that would be about 12000gph plus a wavebox.
Just depends on the size and length of the tank and if you have sand etc.
I think 30-50x is more than enough, anything over that may be overkill... but then again, maybe not based on the tank design.
