Rug Dr. Life Cycle

Simple bottom Question: How many average hours does it take for a Rug Doctor Unit to burn up or become not worth repairing. Anyone? Anyone?

I am a Home Depot Rental Associate. We have over 15+ units that we rent out. To give our customers the best possible unit after they are returned, checked and re-rented, I keep very close eye on the unit time of use for even rotation. Is there an average time line of hours for a RugDr unit from zero hours to end of use.