It takes seven years of college to become a physical therapist. Four years as an undergraduate and three as a graduate student. (Some students take 8 years due to spesnibglongsr as an undergraduate to complete pre-requisite courses while others attend "dual" or "fast track" programs that allow them to earn a BS/DPT in six years.)
Becoming a PT Assistant requires two or three years as it is only an associate's degree.
You can learn more about both careers by visiting this page here http://www.apta.org/prospectivestudents/