A fresh pair of Eneloops last about 13 hours in my Garmin GpsMap 60cx. I prefer to keep the unit switched on when walking. Having a track of everywhere you've been can be more useful than just a few waypoints.

In any case it's worth leaving it on for half an hour or so every few months, so it can update its almanac. (The almanac is a database of satellite positions which helps the unit get a quick initial fix.) It can also be worth switching it on before you start the journey, because they can be slow to get an initial fix when they are actually moving.

Mine is mostly used for in-car navigation. I have found it best to configure the screen "north up" rather than "track up". Although "track up" makes it easier to understand the directions it gives you, "north up" is easier to relate to paper maps and to the real world. Eg it is immediately apparent if you are moving east to west when you should be moving west to east.