Awesome man. She's about 7 in right now. I appreciate you hitting this up so I finally have an idea when I'll see them adult colors. Do these take a long time to grow cuz I feel like she's growing super slow?
Like Tobias has already mentioned, all rays/ fish grow at different paces. There can be a somewhat typical but there are factors to be taken into consideration.
clean water, temp, food- quality, frequency, space to grow etc.
I bought my trio about 4 months after seeing yours on the market. When they arrived they were all 4”. Now my male is roughly 7” and my females are about 5”-6”. I keep their water at no more than 30 ppm nitrates typically and feed smaller amounts 2-3 times a day with a fast day here and there of just 1 small portion.
as fishtanktaco has mentioned, my male is already starting to look more and more like the colours you seem to be after. Even at 7”, but the ones that I see online and am like dam- seem to be roughly 12”.
patience and clean water and your girl will be a nice stunner in no time.