The most common spot for strings to break on bows is right under the serving, where the arrow is nocked. As the strands break one at time, your shooting will start to be a bit off. Then, without warning, your string will break at the most in oppertune time. Sometimes it's quite painful. If you do a moderate amount of shooting, I'd change it every other year. If you do a lot of shooting, I'd change it every year.

Good luck