I think it stays on all the time although you can turn it off. I didn't notice any battery drain. There are various interest settings that can be toggled.
For example, when we went to a mall in Dallas, it notified me of various store information, which I didn't find very useful - it wasn't like store websites though, but various blogs about brands and stuff. Still, I didn't need it for walking around the mall. But when we went to the missions in San Antonio, it would pop up (notification) various features of the mission as I approached them, i.e., the Rose Window at Mission San Jose, and various other points of interest in the mission. I understand that if you are using headphones you can have it read the info to you as well. kind of like a guided tour. Downtown Dallas, it alerted me to numerous points of interest that I would have missed entirely. It alerts for photo spots nearby. It alerts for interesting restaurants, even if you are just driving in the area.
Here is an example of the kind of stuff that it comes up with