There used to be an expiration date on the front of the detectors. Now the ones I have seen have a statement that says replace after 60 months in service. I'm on my third one in 4 years of full time use. I would be surprised, based on my experience, if any of the make it to 60 months.
And when mine failed they both did it at 3:00am. Go figure.
2006 GMC 2500HD CCSB 4x4 Duramax/Allison, Titan 52 gallon fuel tank, Prodigy Controller, B&W Companion Hitch
2010 Jayco Designer 35RLTS, Cummins/Onan RV QG 5500 EVAP, Progressive Industries EMS-PT50C, TST Systems 507 TPMS, RV Flex Armor Roof