Numion has several excellent tools in order to estimate how long it takes a file to transfer on a network or to be downloaded from the internet. Of course these are estimates that are affected by overhead and other factors.
For example, if you have a 2.3 Gigabyte iso that you want to download and you don’t have anything on your network that is using traffic , you can estimate 4 hours and 19+ mins on a T1 with a 20% overhead.
You can download the calculator page and use it offline (Noted on the site).
Other calculators include stopwatch to measure your website loading speed, server requirements, server capacity and others.