If the water in the crock rises at a rate of roughly 1 inch per minute, you are collecting 60 gallons per hour. Another way to get a feel for needed capacity is to track pump cycles over an hour.

The sump pump monitor comes with a Flo by Moen Smart Water Detector that can be placed near the pump, but outside the crock to detect leaks. This one is set the drain pan of a water heater.