I would say as a minimum, 10 times the tank volume per hour, but 20 times is much better.
There are many that say you do not need strong flow in a FOWLR, but I disagree on several fronts:
1. Strong flow means the LR will "work better" as a filter.
2. Strong flow keeps detritus in suspension where it will make its way to the skimmer and get bronken down / drawing out.
3. Good flow will improve oxygen levels and result in better pH etc.
4. Good flow will keep your fish fit and healthy, rather than fat and lazy.
There are other reasons too, but they are the main ones......
Add up the flow rate of your powerheads, and aim for 20 times...... you can get away with 10, but 20 is my recommendation, and 30-40 times is perfect!
HTH
Matt