I would imagine that most people replace them whenever they start noticing performance degradation/failure. I've been using my Herpstat Pro for one year now, so I'm afraid that I can't give you much of a longevity reort.
Helix and Herpstat are pulse proportional thermostats, meaning they have no mechanical relay (i.e. no parts with friction on them that could wear out) so I can think of no reason to change one out that is performing normally, no matter how old it is.
Helix and Herpstat are pulse proportional thermostats, meaning they have no mechanical relay (i.e. no parts with friction on them that could wear out) so I can think of no reason to change one out that is performing normally, no matter how old it is.