How much TAing is allowed or required depends on your field and department. I'm in a statistics department that expects PhD students to TA every semester (except their first and final year). It has taken me some effort to weasel out of around half of the teaching appointments, since I find teaching (especially grading) quite time-consuming, while industry internships both pay better and generate research experience. On the other hand, people I know from the CS department only have to teach 1-2 semesters during their entire PhD.