An Offtopican becomes an offtopican simply by existing and being accepted into the community and posting. There is no real test, post count requirement or number of hours you need to clock up looking at OT.
You are kind of an Offtopican. One of the few posters I'd say who regularly post and are't offtopicans are Ewok (he's lolpermabanned anyway I think) and skyline.
Its rather undefined, considering how in a way its 'special' to be an offtopican.
And it just happens really. Its like... I can't describe it, but its one of those things which happens over time without noticing. Its like going from childhood to adolescence, there is no 'YOU ARE LEAVING CHILDHOOD' sign, no 'YOU CAN FAP NOA' sign, it just happens and by the time you've noticed its already happened.