OP here. Thanks for the responses. I should've mentioned that he's not necessarily looking for more pay (though that would be nice!). I think he's just wondering how much longer they can force him to act as a supervisor when he would prefer not to be one, if that makes sense.

There is no official time when they would have to make him a true 15. I've known people who have "acted" for 2-3 years.
It can go on indefinitely.
A desk audit can help, but it can also burn him in that it can be viewed as being whiny. (Tread carefully when requesting an audit.)

Your DH should check with his union.

Ask for a desk audit?

I think it is 90 days and then they have to pay you. Even then you can only "act" for so long and then the agency gets themselves in a bind (ie. may have to non-competatively offer him the job).

My husband isn't a supervisor but has been acting as a division director (GS-15) since December with no end in sight, thanks to the hiring freeze. He's still doing his own job as well, with no temporary bump in pay. How long can a collective bargaining agreement position act as a supervisor?