When it drops into the dim incandescent mode, it will not run very long at all, maybe a few minutes I'm guessing? And will progressively grow dimmer, and dimmer quickly. However, the beautiful thing about the A2 is the LED's at this point will run for a very long time.
Now that I think of it, for some weird reason, the LED's themselves actually have 2 levels of brightness whenever one's cells are depleted enough that the incan lamp won't fire up. When one turns on the LED's at this point, if you fully press the tailcap switch, or full tightnen the tailcap, the LED's will gain a little brightness. I don't know why it does this, but both my A2's do this, and I've seen other comment their does it as well.