They probably don't ever stop growing just the growth slows down considerably. Usually they should grow giving the right conditions to about 11 inches within the first year and then slow down probably growing an inch or 2 every year.Having being told the average size of the the common pleco me question is at what age do they stop growing at , Minne's 10 inch at 1 & a bit years old .
Thanks yet again FishyJake13 , by the way found a tank and equipment 237 gal £1320 altogether halfway there with the dosh
Won't tell my misses the price she'll blow up