About 5 years ago, when I started getting more serious about lawn care I paid ~$75 for a jar of tenacity. My yard is smaller so I've made it about halfway through the jar, and in hind sight the signs were there, it's dead Jim...
First year, one application at 4 oz./acre took care of my weeds. By about the third year, the first application just didn't quite kill off the weeds but the second application would finish them off.
Finally this spring, I notice I've got a large portion of my yard taken over by creeping bentgrass. So I hit it with a 4 oz./acre and planned on doing 2 oz/acre every two weeks after to stall the bentgrass. But after the first application, the weeds didn't even bleach, and they were still fine after the 2nd application.
I decided to replace my tenacity and not only has the price almost halved, but one application at 2 oz./acre and the entire yard has frosted tips, the bentgrass is stalled and the other weeds have vanished.
TL;DR: Tenacity will start losing its potency around 3 years, if yours doesn't seem to be working, consider replacing it.
Now if anybody knows what happens if you get close to max amount per year, please let me know. I'm curious, because now I have no idea how much active ingredient actually made it onto the ground.