I actually found a paper that discusses the maximum age to size relationship in reef fishes from the Smithsonian, which on page 68 states that “The genera
Acanthurus,
Ctenochaetus, and
Zebrasoma displayed rapid initial growth, … resulting in the characteristic square growth curve” (Choat & Robertson, 2002). There is an accompanying figure on page 69 that actually shows
A. xanthopterus on that square growth curve. The figure has the fish growing rapidly to a standard length of about 16 inches (so probably 20 inches total length) over the first 10 years of life in a fairly linear fashion, so I would expect about an inch and a half a year in an aquarium setting. Since it’s about a foot now, I would say you have a decent amount of time with it, maybe 2 years, before it gets to around 15” which is when I would say it’s pushing your 300.
Link to the paper:
https://repository.si.edu/bitstream/handle/10088/6900/Salebook_chapter.pdf?sequence=1&isAllowed=y