Actually, after the first TB injection, you have to get it rechecked within 48-72 hours.... after that, it's useless.
But back to your original problem, I hope my comments help. It's really all about setting limits and STICKING TO YOUR GUNS. If you let them run you ragged, they will.
I think a lot of the time, they're worried / afraid that they won't see anyone for hours and hours, so they try to hold onto you as long as possible. If you can explain to them that you will be in their room every two hours at a minimum, and that you'll stick your head in the door in between then as often as possible, and then you follow through with your promise, you will hopefully see positive results.