A. On average 3 to 6 months or when you reach a 10 to 12 ply fit. The preparatory is used to shape the residual limb as well as strengthen the limb. Usually during this process, the limb changes and begins to shrink down in size. The first definitive device is usually a glorified preparatory due to the limb still changing. Most changes occur and level off after the 1-1/2 years.