Depends on how much they eat and what your feeding them, I had a rather large snail nearly double in size in 2 weeks while others are still only 1/3rd his size.
When I put the parents into the tank, they were both the same size. Then the ivory(mother) grew very quickly. She has put on about an inch and a half of new shell since april. The blue(father) has only put on about a half inch. Dont know what the difference is though.