They use regular AA batteries, and those can last up to 6 months.
The scent tubes are supposed to be changed every two months I think, but I don't do them that often, just spring and fall but they seem to still be effective.
I have found that they work best when used close together, about 2' apart initially. Especially when protecting a deer delicacy. When the deer have trained themselves you can spread them out, or sometimes use a single in a small area. I think they are on the pricey side, but they are very long lived and so simple that there is nothing to break.
Much cheaper than replacing plants. I think the directions suggest also using a deer repellent when you first install them. If you can keep the deer a little wary to begin with the effect is greater and makes a more lasing impression.