How do I measure the discharge from a Sump Pump?
Time how long it takes to fill a 1 gallon bucket, then divide that time into a minute, that gives you GPM; Gallons Per Minute, a standardized flow rate. So for example if, while it is fully flowing, you put the bucket in the stream, someone else starts a stopwatch and you see that it takes 6 seconds to fill the bucket, 60 seconds / 6 seconds = 10 GPM. 

Then if you want, get what's called an "elapsed time meter" that works on the same voltage as your pump, and wire it in parallel with the pump so that it records how long the pump runs. Multiply that run time (in minutes) during a time period by the flow rate to get a larger averaged flow over that time period, i.e. per hour or per day or per week to make it more useful for deciding what you want to do with it. So for example if, during a given week, the sump pump ran for 3.2 hours, that is 192 minutes, x 10GPM = 1920 gallons per week that you need to figure out how to safely dispose of.