pretty confident they can reach 2ft in their first year, and 3ft by 2nd year. i got my gator as yoy a couple months ago and he's on pace for another 2" growth spurt this month
They grow fast for their first year or 2 but after they slow way down. I've had my boy for just over 2 years. When I first got him he was small, probably about 6-7 inches. Now he's a happy and healthy 27" almost 28
I hear they have a growth spurt during summer so I figure he'll get up to MAYBE 30" by the time football season gets here which is the end of summer.
To answer your question I would have to guess around 4 years to get over 3 feet, but I would also think it depends on the size of tank(s) and amount of food.