The key to string life is keeping them waxed.If you wax your string on a regular basis it will last much longer.When waxing don't be scared to use too much,and rub it in good.
On my 3D bow I usually change strings once or possibly twice a year,but this is because I may have 1000's of shots on them,not because they are worn out.
On my hunting rig I change about every 2 yrs whether needed or not.