The engine mounts in most vehicles do not have to be replaced at regular intervals. In the right conditions, many engine mounts can last for the life of the car itself. In very hot or very cold climates or in harsh driving conditions they may not last very long.
However, engine mounts should be inspected frequently and checked for major cracks or broken rubber. Use a flashlight and a small mirror if necessary to inspect all around the rubber mount. Symptoms of a broken engine mount include excessive engine vibration and a shifting or clunking of the engine itself when accelerating or braking. If you suspect that you have a broken engine mount it's important to replace it as soon as possible. In many cases it can be relatively easy to replace on your own.