I do 6 hours teaching per week during term time, plus preparation and any marking for those classes, plus 6-8 hours of exam invigilation yearly, and I also have to deal with student queries in my own time. I don't get paid for it either, the uni pays my fees in return for my teaching and gives me a 12k bursary to live on (I suppose you could say my 12k bursary is what I get for teaching). It's annoying because other people who aren't funded through teaching get paid a fortune for it, plus a bursary as well, if I was paid for my teaching I'd get over £200 a week on top of my bursary :( But it was the only way I could get funded so I have to live with it. I spend roughly 9-10 hours per week on average doing teaching duties, it would really hinder my PhD if I didn't work really hard. I suppose it's good work experience though, although I still get the feeling I'm being ripped off :(