If you are set on the undergravel filter then leave it on all the time. One powerhead will run the whole thing and will provide a hefty current for the fish. As long as you do regular gravel vacs and take the time every 6 months or so to clean out under the filter plates it can run clean for a long time.
If you run another filter then run the UGF in reverse. Still works fine for biological filtration and relieves the need for cleaning under the plates. A prefilter on the powerhead will keep debris out from under the plates.
Is there any specific reason you want to use the UGF? Many think that they are not worth the hastle and HOB filters will provide more than enough filtration. That and if you want to go planted some day, UGF are not the best to have.