When to tune/replace brake cables for optimum tension
Normally you replace a cable when it becomes too badly rusted, or the cable inside begins to break.  The first condition you notice because the cable does not move freely, and the second you notice because the brake levers "give" (and don't bounce back) when you squeeze them hard.  If not rusty, a set of cables can easily last 10-20 years.

In your case it seems likely that the brake pads are simply wearing rapidly.  With V brakes, for 1mm of pad wear you'll need to take up about 3mm of cable slack.  If you're having to adjust the cable too often you may wish to change out your pads for something harder (though note that this will reduce braking power).