Hey TomJim,
I was in a similar position a while ago in that I was acting up for a while with no definate time scale put upon the acting up period. Employer was in no rush to make a decision on whether to interview for the job or to just leave me acting up. I contacted my union (begins with an 'I' and ends with a 'T') about it who were pretty useless I must say.
I ended up giving my employer an ultimatum (told them I would do it for three more months and no longer without a decision being made) as I was sick of doing two jobs, they relented and held a competition for it. Thankfully I got the job but was all too aware that my bluff might have backlashed on me i.e. someone else might have got the job and I would have to go back to the lower paid job, but I had had enough so it was a risk I was willing to take.
Bottom line is I don't think there is a timeline they have to follow before you just get the job. I would contact your union - keep a paper trail - to verify this (if they can!!) and then talk to your manager or the most senior person you can about options. After that you will be in a better position to make a decision.
Good luck
Rosen