How to find how long a Mac App Store app installation will take
The App Store tells you how long something that you have just purchased will take to download.

Once the purchase has been made, just switch to the Purchases tab, and see the progress bar and the time estimate.

Obviously for large files, the estimate is based on how much more data is left to download divided by the current download rate; so your estimated download time will vary as the transfer of the file progresses.

The same is true for downloads on the Updates tab also.