Hmm, had that discussion with myself when I got the HP laser some years ago. So I made up a test sheet and printed it off - made 2 actually and put one in the filing cabinet and t'other in bright sunlight to see how it stood up. Then when it occurred to me - about every 12 - 15 months I printed another copy of the same sheet. Confirmed that the printer was still behaving reliably and that the toner was still the same quality.
Sadly the engine failed after some years - but by that time the printer owed me nothing.