if you dont have a 50g or bigger i dont suggest ever looking into oscars. i tried doing exactly what your doing and i ended up spending loads of money and getting myself in way over my head just because i didnt want a goldfish haha. so id really think twice if i were you
to put it in perspective. i started with 2 tiger oscars (1 inchers) in a 30 and 7 months later i now i have 5 fish tanks of an accumulative 250g lol so realllllly put some thoughtt into what fish you purchase, and make sure your budget and dedication coordinate. good luck though bud
id say a few months. if i was you i would think about getting a 55 but preferably a 75. i got my oscar about a year ago at 2-3" and its about a foot now. and moved for a 75 to a 135 cause i felt the tank was to small.