As a golfer the only part of the club you are in contact with is the grip. It is extremely important, often overlooked, and inexpensive way to add some performance to your equipment and your game. However, the grip needs to be in good condition, the correct size, and type.
Factors to consider:
Condition of the grip.
Worn grips cause you to squeeze tighter to control the club and keep it from twisting in your hands due to the lack of traction they provide. This puts tension in the hands, wrist, arms, and shoulders all leading to a loss of distance and control.
Correct grip size.
The wrong sized grip can also affect your golf swing. Too small of a grip can cause a quicker release of the clubhead, making it easier to hook the ball. Conversely, too large of a grip can slow the hands and make it difficult to square the clubface, making it easier to slice the ball.
Correct type of grip.
Grips are all about feel. You need to be comfortable with the material, texture, and style of your grips. Don't forget about the weather conditions you play in. Hot and humid conditions where moisture and sweat can be issues may require a different texture or style.
How often should they be changed?
Grips are good for about 40 rounds of use. One practice session is equal to one round. If you play regularly, then once a year is good. If you play two - three rounds per week then twice per year.