My thoughts (based on my own soaps) run much along the lines of what TOMH said- soap can get better and better for a loooong time, but as the good Gent said, there comes a time sooner or later where things flatten out and the improvements are less pronounced/noticeable and aren't much worth the longer wait.
Just exactly when that point of diminishing returns is reached will be different for each soap-maker and their soap, based on their individual formulas and their personal perceptions about them. For what it's worth, the point of diminishing returns for me and my own soaps varies from about 3 months to a year, depending on the formula.
Edited to add
, I still have soaps from about 10 years go that are still in excellent condition, although the scent has faded. It's always a huge treat for me to pluck one out and use it.