Clearing up two things above:
SPOT will last over a week even using tracking, if you start out with new lithium batteries.
Using an Iridium-satellite system is just as unreliable when you are in deep canyons. I tried a satellite phone last trip (a friend has one), and it would barely connect in the Kern River canyon, and when it did, the connection lasted only a few minutes. So any device using satellite, no matter which system, will have trouble intermittently in a canyon. In areas with more sky visible, they work great -- 100% of the time on a mountain summit.