very slow growers. they get more girth than length. I have one nearly 2 years old that is 4", and one double that age that is still only about 6" but FAT.
I power feed all my plecos to thy and get them to grow fast, but about 1/3 of the species grow very slow like royals. I have a dozen royals and relatives from 6" to 14". Mine grow about 1.5" per year to 6", 1" per year to 8", 0.5" per year to 12" and VERY little after 12".
I got mine around 3 years ago. He was around 1.5". He is now 6-6.5" this seems to be the standard growth rate of most panaque species. There will always be exceptions though.