It really depend on how often you shoot and how well you take care of your string. I shoot enought that I change my strings out once per year.
If your string is showing any sign of wear, or you are having problem with peep rotation, change them out. I am sure that after 3 years on a stock string you have have some stretch going on, so moving to new string is not going to negative impact.
Take a look at Vapor Trail's strings.
www.vaportrailarchery.com