I've got the Viridian Reactor 5 green laser on my Shield 40. Viridian advertises that the batteries last up to 1 hour of use. So far that seems to be pretty accurate, at least for me. My experience has shown that the laser remained consistently bright right up until the time that the batteries started to fail, at about an hour or so. Before you get to that point, the low battery warning light will begin to show. Once the batteries start to weaken, then the laser will dim, or go into the "strobe" setting.
The battery life on the Viridian laser is really my only criticism of the laser. Other than that, you don't even know that it's there, and it holds zero well. If you choose to use the laser all of the time this could be a problem, but for me, I only plan on having the laser turned on when it is holstered. Then if you would happen to need the gun in an emergency, the laser will automatically come on. During regular range use, I will always use the iron sights, turning the laser off and conserving battery power. The time that the laser is especially effective, is in low light or dark conditions.
Dealing with any laser or sighting system that uses batteries can be a problem. That's why I feel that the operator should be well practiced in using the regular iron sights. But for low light conditions, or in close quarter conditions where extending your arms to a full shooting position could lead to someone grabbing the gun, the laser is great. You just have to learn to deal with it.
