Most routers have a plunge depth of around 2-3 inches. Some routers, however, can plunge much deeper. The Bosch Colt Variable Speed Palm Router, for example, has a maximum plunge depth of 5 inches.
When you're using a router, it's important to keep the bit at the right depth. If the bit is too high above the workpiece, it will create an uneven surface. If the bit is too low, on the other hand, it may cause the router to overheat or even break.
The best way to adjust the bit depth is to use a router with adjustable speed and feed rate controls. This way, you can slowly lower the bit until it reaches the desired depth.