Nobody that I have asked has been able to tell me the 'Normal' LIFESPAN' of a modem.
Depends on the specific product. IME, the ethernet ports on a Netgear DGN2000 fail before the warranty expires. I had several replacements sent.
I eventually got a DGN2200 foc from Plusnet which has kept going for very nearly 5 years, powered up all the time. It is used as a switch and wifi access point rather than as a router.