I had a common in my 75g years ago, and by 18 months(ish) of age he was 9.5 inches long and seemed quite unhappy as he really didn't have room to zoom around like he wanted to. I traded him to someone with a pond, and he went happy crazy when he was put in it.
My personal opinion, as a goldfish keeper, is that anything under 100g for single-tailed goldfish is just not enough for the long term; it's great for a few fat fancies.
