A LENA device should last about 5 years, or 500 day-long recordings, when used under typical conditions.  

The device, including the battery, is covered by warranty for one (1) year, during which time the device is eligible for replacement. The failure rate even beyond a year is quite low.

To maximize the life of the recorder, avoid prolonged exposure to extreme temperatures, which can affect the battery. Do not submerge it in water.