really, it depends on the fish. Like I said, I had one that slowed down around 14" so he would last longer in your tank then the ones that don't slow down until 16". In Another 6-8 months he should start slowing down in growth until you won't even notice him growing. If he gets to 16", I'd say its time to upgrade, if not, you'd have a good year or longer until you really need to think about it.