TA Funding Length in STEM Field in USA I understand that depending on the department and the school the duration of the TA funding for a Phd student can vary. However, I’m interested in learning whether it’s common for departments in STEM field to have less than two semesters of TA funding for the same student in US.
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.