Go to Speedtest.net after pausing the ML download and see how fast your download speed is. Of course the result also depends on the load that Apple servers are under and anybody here can only give you an approximation.
Another way to work it out is below;
You should I'm assuming know what your average download speed is by looking at the downloads window when you use a web browser as per the below.
What you need to do is look at the total size of the file you are downloading, for ML it is I believe 4.07GB or 4,070,000KB. Take the number of KB and divide it by your download speed to give you the estimated duration in seconds. You can then have an idea how long the download will take assuming a constant speed.
For example mine says 215.7Kb/s and the download size for Parallels 8 is 318.4MB, (or 318,410KB), so 318410 / 215.7 = 1476s, or about 24 minutes and thirty six seconds.
The download actually took 24 mins and 58 seconds according to my stopwatch.
The trouble I think you have is that downloading from the Mac App store at least in my experience is quite spotty, sometimes fast and consistent and others not so as you are experiencing.
Chamars speed above is ridiculously fast.