Well at least you have some sort of plan for him, and i couldent really say it depends on his growth, ive found out that if you power feed them witch i dont suggest, he could be 2 foot in 8 months or less but if you feed him every 2 days he wont grow stupidly fast and i personally think its healthy for the fish and makes them more active aswell, then he could have about 18 months in the tank. but thats my look on the growth , all depends on the feeding that u do.
Dav.