It is really going to vary with each piece, so it is hard to say definitively. The white strands are typically the ones that any change becomes most noticeable on. They seem to be the most susceptible to over-treatment. Have you ever seen the whites that become chalky looking? That is a classic sign. In an over-treated strand this can happen within 3 months. In a properly treated strand, a strand coming from a factory that is not working towards the largest, fastest ROI (few and far between), the changes may hardly be noticeable.
This is why care of the pearls is also so important. If the pearls are cared for properly, while they will have slight variations over time, they should last a long, long time.