How do you differentiate the hub counter on a cassette counter with time. Thus forming an equation to exact the time remaining via the tape counter.

Are you sure you don't mean integrate? I don't totally understand the question, can you make it much clearer?

Such a relation is surely not linear, depending on the thickness of the tape, what the counter is really measuring, etc. Your best bet (I've actually done this) is to let the tape run and record the counter with the aid of a stopwatch.