The answer largely depends on your budget. Under optimal circumstances, you should change a batter head when you begin to see pitting, or the film begins to deform, or you begin to hear a buzz, or the head loses its resonance, or any other indicator that the head is beginning to lose tone. However, many drummers need to stretch the life of their heads for financial reasons, which often requires more frequent tuning adjustments or some form of muffling to disguise offensive buzzes and overtones. The worst thing is to wait to change a head until it breaks. That typically indicates that you have played the head long beyond its intended lifespan.