I get 1/2" a month from some of my acros and montis. I consider that fast. I got a very genorous piece of tan acro not sure of the exact name, from nate on dec 15th or there about and it has exploded. Grows super fast. Has grown at least a 1/2" from every branch from day 1. Didnt take even a day rest from being fraged and moved to my tank till it started to grow. My milis dont always grow as fast but I believe them to be capable. One of my milis, a green hairy mili comes close to 1/2" each month. As far as lps Id say its not hard for them to double in size meening each head split into two every month. I however am not sure how long actually it takes for my lps to split. They split fast at times and not so fast at other times.
I got a 3 headed hammer frag about 5 months ago and its now 14 heads.
I use that as my guide and if I see no or little growth then I move it to a new location and see how it does there.