I've not found them to be deranged monsters. They WILL eat anything kept with them that they can fit in their rather cavernous mouths. They WON'T go ballistic (ala Mystus wyckii). I still wouldn't keep anything you value with them, as they will easily attempt - and succeed - to eat fish their own size.
Your real question ... if fed properly (to satiation every day), most of the Bagarius species should outgrow a 55 in less than a year. A lot depends on how you define "outgrow". I would consider it outgrown at 12", which it should reach in 6-9 months, IME. That would also depend on the temp you keep them at; higher temps cause their metabolism to increase markedly.
