I am looking for a way to display on a single line, a running countdown for a given amount of time in a terminal using a bash script.

I am looking for this to use as part of a larger bash script that captures Video. The script sets up a bunch of parameters for DVgrab, and one of the parameters set is the tape length. I would like to use this $tape_length as the input for a running countdown so that at any time during capture I can see how much longer the capture will run for.

On a side note, in my script $tape_length is entered in the form hh:mm:ss

I feel like this should be easy, but despite some extensive searching I can't find any leads or even scripts to steal and modify! Any help is much appreciated, I'm very new to bash scripting...