TA Funding Length in STEM Field in USA
I doubt that you will find many restrictions. It is fairly typical in many fields for students to spend a few years studying and paying for it as a TA. It is, normally, a job-like position, not a grant. 

There may be some places that put a limit on it, but I doubt you would find anywhere that it was as little as two semesters. As long as you do a good job, of course, and keep up with your studies. 

On the other hand, admission to a doctoral program doesn't automatically imply that you have a TA position. I is a separate transaction. But once you have the position, and perform well, it can continue.