Well, switch height is more of a preference than anything, although the unwritten rule is to have the switch at a good working height. That height might be different for a normal, everyday family, but for a disabled person in a wheelchair, the height is much lower, allowing them access to the switch. Typically, in these areas, switches are set at 48 inches to the top of the switch box. In a standard home installation, the height might vary from 48 to 52 inches from the floor to the top of the box.
So you may be wondering why this height is standard, right? Well it has to do a lot with drywall. You see, a typical home's walls are around eight feet tall and drywall comes in four feet wide sheets. This means that typicall two sheets are laid on top of each other to cover the eight foot wall. Although the lengths differ from 8-12 feet, the width is constant. Therefore, the center of the wall is approximately at 48 inches.
The switch box can start at the top or the bottom of the 48 inch mark or centered on the 48 inch mark. Any of these choices makes cutting the box opening in thee drywall easier. This is why I choose to follow this method of marking switches based on the 48 inch mark. Normally, mine are set at 52 iches to the top of the switch. This allows the drywall installer to cut the switch opening out of the top sheet and the bottom sheet will flush out with the bottom of the switch. Believe me, your drywall installer will thank you for making his job easier.