Doctor Boris, they limit you to 10 cartridges a month, and you get $2 in store credit for each one, not cash on the spot. I'm unclear if the credits are issued monthly or quarterly.
You're going to be returning cartridges for more than 8 years unless you also go to Office Max and Office Depot. Then it will take a little less than 3 years.
What if you go to multiple Staples store, could you not trade in 10 at each one of them?