You should have a check valve unless the end of the outlet pipe is below the sump pump and not susceptible to flooding.
Then figure out how how much water you are pumping on each cycle. Apply old high school math to figure out the area of the pit and volume pumped per cycle. In the same way, you can figure the inflow rate.
Some pumps have very small range of start/stop because of the built-in switch. You might want to enlarge and deepen the pit, and put in a tethered float switch to allow a larger adjustable range.