During a regular semester (16 weeks), undergraduates enrolled in at least 12 hours and graduates enrolled in at least 9 hours are considered full-time. You have to remain a full-time student to receive full benefit.
Classes that only go through part of the term will affect your rate of pursuit. For example, if you sign up for 12 credits for the 1st half of the semester, you will only receive benefits for the 1st half of the semester regardless of how you are classified by the university. Once your classes end, the VA considers the semester over. It is especially important to remember this when you are registering for summer classes. The VA will calculate full-time status based on where dates overlap.