The TrekGuide uses two lithium-type CR 2032 batteries. These are widely available at hardware stores, drug stores, etc.
The TrekGuide does not have a power off switch.
How long the batteries last depends on things like the temperature and battery brand, as well as TrekGuide use and functions used. The digital compass uses the most power followed by the barometer and altitude (altimeter). Under normal use and conditions, the battery should last between one and two years. The TrekGuide will automatically switch to the lowest power function, altitude, from the digital compass mode or the barometer mode after a few minutes of rest in order to save power.
If you don't plan to use the TrekGuide for a while, it is best to remove the batteries to further prolong battery life and prevent corrosion inside the unit.