Just a guess, because you don't say what's displayed when air time isn't:
I notice you're nitrox certified. I'll guess you've set your Cobra to Nitrox mode.
In Nitrox mode, the %O2 is displayed instead of remaining air time until the remaining air time is under 30 minutes. See the manual section 3.4.2, page 29, middle of page.
If that's not it, what is displayed when air time isn't?
neutrallybuoyant, the air time goes to zero at 500 psi, not zero. Also, the time is deliberately pessimistic with regard to changes in consumption rate; dropping fast as consumption rate rises, increasing more slowly as consumption rate drops. See 3.3.3 on page 21. In general I agree, I use PSI mostly, but I look at the air time to see if there's something I don't expect that I should focus on.
As for fnfalman's comments, could be, although it sounds like a couple of minutes is all that you'd expect. The manual (3.3.3) says
Anytime you enter into a dive, the remaining air time calculation begins. After 30 - 60 seconds (sometimes more, depending on your air consumption), the first estimation of remaining air time will be shown ...
"... either not working or began to work very late into the dive." doesn't sound like that, but I guess there could be room to interpret it that way.