A friend of mine brought a coin size shoulder and it grown into a mid size within 3-4 months, although he feeds quite heavily.
Mine was about 1.5-2” and it grown to mid-big size within a year.
Shoulders are really really good grazers, it kept my in-sump frag section in check and half a year ago transferred to my separate frag tank where it now grazes algae all day and started to transform into its adult colors.