Your institution will decide what is considered a full-time course load for the degree program you are enrolled in. If you complete at least half that number of hours, then you will be considered enrolled at least half-time. For practical purposes, the institution will indicate your enrollment status for the half-time test on the Form 1098-T in box 8.
Why do you want to know. There are several tax attributes that are affected by full time student status. Eligibility for the Lifetime Learning Credit (LLC) is not one of them.
The biggest issue is whether you can be claimed as a dependent.
There are two types of dependents, "Qualifying Children"(QC) and Other ("Qualifying Relative" in IRS parlance even though they don't have to actually be related). There is no income limit for a QC but there is an age limit, student status, a relationship test and residence test.
A child of a taxpayer can still be a “Qualifying Child” (QC) dependent, regardless of his/her income, if:
Enrolling in August (Aug-Dec) usually meets the 5 month rule.