Hello there
As everyone mentioned it really depends on the quality it was uploaded in, bitrate, etc. It may also depend on if you are using flash or Youtube's HTML5 viewer too, but not sure. For comparisons sake, we've connected an iPad to a HughesNet gen4 system in the lab over wifi and played Netflix HD videos non-stop during work hours until we exceeded our allowance, which took a few days on the 10GB plan.
Thanks
Amanda