I have two Nest Protects (and a thermostat). One was a gift when they were almost brand new, the other one was purchased several months after launch. They are the battery operated (unwired) models. They are still working as well as the day they were purchased. I leave the path lighting feature on, and the one upstairs (high traffic area) has needed to have the batteries changed once.
If there's anything else you want to know, I'll do my best to answer.