How many hours do PhD students work as a TA (total)?
So are you saying you will only be TAing for 10 hours weekly? And you are unhappy about that?

This work load depends on the university. Some schools are notorious for loading graduate students with lots of TA work. Where I went to school, I had to TA for 20 hours full. 

So if you are only required to work for 10 hours (and are getting paid for 20), if I were you, I wouldn't complain about it. -:)