Your pump manual should have a pump head curve in it which relates head loss to GPM. Your head loss should be between 45-65 feet of head from which you can get a possible range of GPM.

If you want a more exact number, you will need to calculate the head loss for your plumbing. One way is this:

Head loss = Return Head + Suction Head
Head loss = Filter PSI * 2.31 + Suction (in mg) * 1.13

If you don't have a vacuum gauge, use 10' as a guess for suction head.

Also, the most I run my pump is 2 turnovers or 8 hours.