It depends on the fishes growth rate. I would imagine females change faster than males, and it's also largely dependent on metabolism. Higher water temperature can increase the rate of change, as can increased feeding. I don't think it so much a time thing as it is a size thing. Four inches is suppose to be when they begin changing. We had customers report changes in as little as 2 month to as long as 6 if i remember correctly. It also depends, of course, on the age of the fish when it's bought from the LFS.
AKA: Who knows....