I have a z7 Extreme. Got it Dec 2010. hunted 1 1/2 seasons (only shot three times while hunting); generally shoot couple times/week throughout summer; most every day in September; and couple times a week during hunting season. My strings still look fine. It doesn't seem to be shooting any differently than it ever did. How do I know when I need to re-string? Thanks in advance for any thoughts.

Alex