Some golfers update their drivers every year, and others wait fifteen years for an upgrade. Truly somewhere in the middle of this range is the perfect amount of time to upgrade your driver. Unless you are a lower handicap player who can feel the differences between clubs, every four-six years is a perfect amount of time to upgrade your driver.
You may feel slight differences from one release to the next, but you must ask yourself if it is enough of a difference to spend another $400-$500. If you can afford to do so, changing your driver every three-four years will keep you very up to date on all of the latest advancements.