In addition, Golf Pride recommends: "As a rule of thumb, you should regrip once every year. Regardless of whether you play golf every day or twice a year, ozone, heat, dirt, and oils are constantly at work breaking down the materials that make up your grips."
However, I've been able to extend the life of my golf grips by simply sanding them down when they start to get slick with medium grit sandpaper to reveal a fresh layer of rubber. As a reference, Fred Couples uses 220-grit sandpaper. To get them extra clean, follow the sanding with a wash using dish soap and rinse with water. Try it and discover how a little sandpaper can prolong the usefulness of your grips and save you some major coin!