Red DEvils/Midas get up to 15 inches.....I have a raised 1 inch RD in a 55g to 8 inches in just 9 months....Bigger the tank the better...I believe a 75g is good enough for 1 RD to flourish.....Make a showcase out of it by not having any other fish in with it......Do two 35-40% water changes per week....Feed it a variety foods from Hikari pellets, Krill, to cocktail shrimp.....
Mine has stopped growing at about 10-11" but is a female. How long it takes to grow depends on frequent water changes and temp as well as water changes. I don't think it's healthy to grow out too fast personally. About a year to 1.5 for mine.