I know a few keeps lazy enough to apply OAV Once in the fall and once in the spring, and they seem to be getting away with it. I know keeps using it four times in a row...
My own personal experience is that if the correct amount is used, and fully vaporized using the three treatment skip method, I usually have a 0 mite count a month later.. So make sure of the amount, Make sure it is not vaporizing too fast, or that you are not dumping some of it out when you pull the pan.
Doing a treatment does not INSTANTLY kill all of the mites, it will kill them over the course of three or four days. More mites will emerge from capped cells and they need to be dealt with. If done at day o, seven, skip and 21 you will catch the vast majority of them and should not have a high count. if you do, something is wrong and needs to be fixed.
I have become so confident that I seldom check all the hives. Checking three or four of them to insure their counts are low to 0 is all I do any more.