While there is no progress bar available, one potential way to estimate the progress is through Terminal, with the following command:

ls -lhrt ~/Library/iTunes/iPhone\ Software\ Updates

It will show the size of the current (.ipsw.download) and all previous (.ipsw) update files, which can be used to estimate progress. For example, for a 2.9G update file (can be estimated using previous update files), if we see that the current update file size is 2.3G, then the progress would be approximately 2.3G / 2.9G = 79%. By running the above command periodically, we can somewhat know how long it may still take (or at least the downloading is still in progress and not stopped because of network or other issue).