Watching Tv Series online :O how illegal !
Buuut everyone does it including me :)
Okay sooo a half hour show... Depending on the website the original embedded video is streaming from and the quality of the video itself, your looking at anywhere between 70,000kb (70MB - low quality) to about 150,000kb(150mb - decent quality).. Rarely get any good quality ones without downloading the whole file haha.
There are 1024MB in 1GB, but your ISP probably just gives you 40,000MB and calls it 40GB lol, so your looking at roughly 260 decent quality videos (assuming you only use your allowance on these videos, which is impossible lol.. every photo, video, and even just webpage you see/visit uses some up).. So i'd say your looking at about 250 episodes. You should still have a little while before you go over your allowence.
You can always check it though. I don't know what ISP you're with, but I'm with Optusnet and i can just go to http://memberservices.optuszoo.com.au/login/?target=/myusage/ and log in and check how much i've used.
You should be able to do this with any ISP
Hope that helped haha