Directv does compress a lot of their streams, so they may end up smaller than this. If you've recorded broadcast mpeg2 streams, they may be as high as 18-20+Mbps.
Theres an easy way to measure this out for yourself. Get a copy of directv2pc loaded on one of your computers, and load a sidebar app, task manager or other networking app that measures your total network throughput on your computer. Fire up the network app and get a graph going and minimize it. Get directv2pc going and start several different HD programs playing. Pull up the graph and watch the total network usage to get a feel for it. I find mpeg2 football at 1080i gives me a lot of spikes into the 12-15Mb/s range during fast motion scenes and thats my worst case, but I dont record OTA.
You'll get something like this...the video is missing from the screen cap due to directvs "prevent you from doing anything that might let you copy any content at any time" strategy