See my testing file here:
But from your reply, I take it, then, that there is no simple solution within FileMaker itself. Not even a basic display of the number of remaining seconds. Oh, well.
Here's a solution using the script step "Install OnTimer Script."
If the user holds the mouse button down anywhere in the window, the script pauses, so they're on their own honor to respect the timer.
If you want to make a bar that fills or empties, you could look at progress bars like this:
Both solutions look very interesting! Still considering user interface design, so don't know which way I'll eventually go, but good to know that I have viable options.