When to tune/replace brake cables for optimum tension Original Question

What physical symptoms do I need to look for to understand that my brake is out of tuning or the cables have reached end of life?

It is not necessary to consider the brake shoes in this question as I already have a previous question on bicycles SE covering that.

MY USAGE
I ride everyday for work and fun. I cover about 20 miles on average every week but would be doing a lot more than that.

MY REASONS FOR THIS QUESTION

I have been tuning my front and rear brakes by either releasing the nuts near the handle or just adjusting the cable near the brake (i.e. where the noodle is). It seems that they are running out quicker than I though (i.e. running out of tuning). I really don't know what to do anymore. I know that a really bad way to do this is just replace the brake cables, but I just almost bought the bike (November 2012). I think there are better ways of tuning and managing the brake performance. I have attached some pictures for the users to see what I am talking about.







The posts that I found on Bicycles SE and youtube are not really helping me for this. My brakes run out of tuning a bit faster than I thought. If I am doing something wrong, a slap on the wrist will be appreciated! 

TIA

UPDATE SINCE @DANIELRHICKS POSTED HIS ANSWER

I have replaced my brakes whilst you folks were reading this and resulted into the followings now. Could anyone tell me if it seems wrong? Also, I am not sure if angle of my V arm of the brake is correct. Do I need to fix that too?




WHAT I HAVE DONE TO FIX THE TENSION ISSUE


Once the brake pads are replaced. I opened up the nut to the maximum i.e. created the highest tension point. 
Adjusted the cable attatchment (i.e. the attatchment of the cable with the allen-key joint near the noodle).
Slowly closed the nut inside (i.e. loosen the cable tension) so that the necessary gap between the pad and rim can be achieved, also the cable attatchment near the noodle is appropriate.
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).