Internet connection speeds can vary greatly depending on a variety of factors.
You can check your connection speed at http://www.speakeasy.net/speedtest/. Select Washington, DC as the location (this would best approximate a connection to Blackboard).
You can use the resulting download speed (see blue circled area in above example) to calculate approximate download times for files at http://www.numion.com/Calculators/Time.html.
Using the example transfer rate of 79 KB/sec in the download time calculator, a 100 MB file would take approximately 3 hours, 50 minutes to download (the rate of 79 KB/sec is about half way between 64 and 128 Kbps on the calculator).