It's a beam; it has to charge up for the same time it fires. I takes 10 minutes for it to completely kill a fully sentient creature, but it also has to charge up for 10 minutes. In the midst of a battle, 20 minutes is a very long time. Of course, the beam has only been used from orbit against a planet's surface, so actual range is unknown. And it's directional; it isn't all around the ship, just from the bow. 

So for Humans and the like, you have about 15 minutes to live if they designate as an extinct species. 5 minutes will erode your neural functions to the point where you'll stop feeling, and your thoughts will stop connecting with muscles and the like. The next 5 minutes are just erasing the really hard-to-destroy stuff- instincts, ingrained habits, etc.

However, they aren't very inclined to use it, and if someone were to attack a certain moon in their home system, then they wouldn't be able to manufacture them again. Too dangerous a weapon to have in multiple hands.