Growth rate depends on many factors - tank size, quality of food, frequency of feeding, water temperature, frequency of water changes, etc.. In general, they reach sexual maturity between 2 1/2 to 3 years.
I started out with 2 about an 1" also in March and it took close to 5 months for them to get above the 2 1/2-3" mark. I have one Regular male about 4" and an Albino female at 3". They are in a 29 gal and have breed 4 times. As Joe stated above their growth rates can be impacted by those factors he mentioned(quality food, water changes, stress, and temperature). I have found that I am able to trade them into my LFS about every 4 months, which puts them close to the 1 1/2" to 2" mark. They are great Plecos to have and easy to care for.