Considered, by who, to be full-time? I think for financial aid it was twelve. To keep your status as a dependent to your parents and keep your military dependent ID? unknown. To stay on your parents' car or medical insurance? Not sure it is a factor. Anyway, fulltime is going to be defined by the person or agency whose rules you are trying to comply with. Again, generally speaking, fulltime is 12 for undergraduate, and nine for graduate school.
Hope this has helped!