There isn't a simple answer to this question. There are many factors that determine how long a pearl must grow in a mollusk.

Pearl farmers first must implant a piece of mantle tissue (which is the part of the mollusk that secretes the nacre) into the gonad muscle. Famers also add a bead nucleus to most of the mollusks, that will be the beginning structure of the pearl. (more on this in a future blog) If this operation is successful the mollusk will start covering the bead nucleus with layers of nacre.

At this point, the mollusk (like us) needs to recover from the operation. This period is different for different mollusks. For the Japanese Akoya oysters (pinctada fucata) the time to recover is up to six weeks, while for the Tahitian and South Sea oysters (pinctada magaritifera and pinctada maxima) it is up to three months. In contrast, Freshwater pearl mussels (hyriopsis cumingi and hyriopsis schlegeli) only need about two weeks. 

After recovery, the mollusks (oysters for saltwater and mussels for freshwater...but you knew this) are returned to the regular farm, where the main growth happens. Many factors determine the rate of nacre deposit. Water cleanliness, weather complications, and water temperature are all influences. You can understand how climate change has had and will have a major influence on the growth of a pearl.

The growth rates vary depending on the type of mollusk, also. Akoya saltwater pearls grow at about .3mm a year. Tahitians and South Sea pearls up to 2mm a year, and Chinese Freshwater pearls up to 5mm a year!

Usually, an Akoya pearl will require about 10-14 months for a desired sized of up to 9mm. A Tahitian will grow for 18-24 months for a minimum size of 10mm and a South Sea pearl will grow between 2 and 3 years for a size of 15mm.

And, yes, all of these factors combine to be some of the reasons for the price and quality of a pearl.