From my readings about oscars and experience with other cichlids, theyn should grow about 1/2 inch to 1 inch per month, and I wouldn't really recommend keeping one past about 8" in a 55, especially since they are so messy and also prone to HITH, which can be caused by bad water quality. I'd say you've got about 6 months, but that's just my 2 cents, so everyone can feel free to correct me if I'm wrong. I've seen people keep foot-long O's in a 55 before for years, but they never got much bigger than that, so maybe slightly stunted?