Taken from an online broadband guide:
It's very difficult to say how much bandwidth you might use when it comes to watching online TV as BBC iPlayer, 4 on Demand, ITV Player and YouTube, to name but a few, all use different video encoding methods to compress video and stream it to your PC.
However, we can give you a very good idea which will let you decide whether you need to go for a beefier broadband package or maybe even an unlimited broadband deal.
BBC iPlayer
BBC iPlayer for example is a ravenous beast in bandwidth terms as it automatically adjusts the quality of the video according to the speed of your broadband connection.
Luckily, there is a low bandwidth option if you want to reduce your usage but this still uses around 450-500MB PER HOUR OF VIDEO. If you want watch iPlayer in high quality a one hour programme will use around 650MB of data.
4oD by comparison uses smaller amounts of data but has inferior video quality.