Hmm, I seem to have changed the batteries in my Wahoo RPM (accelerometer-based) speed and cadence sensors after *peers at per-bike stats* roughly 4000km/200hours use over 2 calendar years.
My Garmin HRM is on its second battery and strap, after I replaced them both earlier this year. Hard to estimate the hours of use that's had (I don't wear it for every ride, and occasionally wear it for several hours of not-actually-riding time around a ride), but I bought it at the start of 2013. It seems to be misreading again, not sure if that's the battery, the cheapo strap failing, compromised waterproofing or I'm just not sweating reliably in the colder weather.
I note that none of these devices are conducive to making precision current measurements while in operation...