When it comes to studying, my understanding has always been that it is about the credit point load. That can roughly work out to be number of hours of study, but I don't know how that works out.
That load would be dependent on your course (and may also according to your education institution guidelines - I see a bit of difference around on a quick search). A good way to work it out might be to look at the total credit point load of your qualification, and divide that by the number of semesters you're expected to complete that in. Then check the institution's determination of what is a 'full time load'.
For example (and I don't know if this is correct, so take it as hypothetical): QUT has a law degree requiring 384 credit points in core and elective units. Nominally that is done over four years, comprising eight semesters. That equates to 48 credit points per semester (8 x 48 = 384). So a full time study load is 48 credit points per semester. QUT considers at least 75% load as being full time (because, for example, different subject weights etc. may not allow a full 48 credit point complement). So, therefore 36 credit points and above is considered full time - and anything less is considered part time.