How long before I need to replace a string? How do I tell?

I am a pretty casual hunter. I probably shoot 2-3x a week for a half hour from august to october and maybe once a week for the oct-nov bow season. Maybe 2500 arrows a season.

I shoot a bear element 30" at #69. and I am shooting and ICS carbon hunter with a 125g field tip/steelhead 125.