Expansions are generally done every 4-6 months, depending on the type of device placed, and the child's condition. When expansions are done more for correction purposes, they can be done more frequently, and as the child gets to the point where they are done to accommodate growth, the time in between can be longer. Either way, the doctor will let you know what stage your child is in and what their decision is.
When a child is expanded (also called lengthening) it is generally done in .5 cm increments. While this may not seem like a lot, it is often sufficient. The expansion places strain on the area. A child who has multiple devices placed, and has them expanded at the same time, may experience more pain than one who has fewer devices, and not as long an expansion.