I would imagine that progress bars generally represent the progression of the task state and not time, for that very reason. Or is that not the case in practice?
It is, but traditionally progress bars were often paired with labels showing estimated remaining time.
That said, back in DOS era, this kind of thing was much more straightforward because most operations that would warrant a progress bar involved some kind of disk I/O, which - if you amortize it - is fairly linear, so one can estimate the completion time relatively well. In more complicated cases - e.g. Win95 installer doing things like hardware detection - those estimates were often wildly off.
