i got mine around 3 inches about a year and a half ago or so and hes already 6 inches or so hes huge...in my opinion it depends on how much u feed them cuz now with two other rbp that are smaller they easily can eat 15 to 20 feeders a night wen their hungry
before i knew about this site i had found riverwonders.com unfortunatly thats where i bought all three of my rbps but they have turned out nice and its between a three,four and 6 inch that they eat 20 feeders per night so three rbp not just one lol thatd be crazy
If you mean adult size (so around 12), 6 years could be a reasonable estimate, but it depends on many factors (diet, genes, water quality, stress levels, overall health, etc.)
If you mean full size as in full size, the answer is never - if they have enough space, fish continue to grow during their entire life. It might take 5 years to grow an inch once the fish has reached a monster-size, but under the rigt conditions, the process doesn't stop.