From the Official Apple FAQ:
Is the sensor battery replaceable? How long does it last?
No. The sensor’s battery has a life of over 1000 active hours. The sensor sends a low battery signal when there is around 2 weeks of life remaining (based on usage
pattern), indicating that the sensor needs to be replaced.
Most Nike + iPod runners and walkers can just drop the sensor in their Nike+ shoes and forget about it. When inactive, the sensor enters stand-by mode to save battery
Monday, August 06, 2007
Nike Plus sports kit battery life
My first iPod Sports kit sensor battery ran out recently. The Official Apple FAQ says 1000 hours. Mine went at about 950 miles, so at my current rate of about 20 miles a week average I'm going to need a new kit every year... Its like a subscription payment to Apple now then. Still, theres no better gadget for runners.