Under what condition will you replace a rubber?

Seems like a simple question but I have been playing all my life and I am never consistent about this.  Sometimes I wait a long time, like I'm afraid to change the feng shui of my blade, and sometimes I change it as soon as it starts to show any sign of wear where the pimples beneath start to be come more visible. These days I am leaving the rubber on the blade longer, not because of the money, more because messing around with my racket stopped being fun a long long time ago and I am just too lazy.  I have discovered Tenergy still plays reasonably well when it stops looking like it would play well.

I guarantee that on this thread you will hear a bunch of widely different opinions.    Each is correct because it depends on so many things, what rubber you use, your level of play, how much money you feel like spending, how much you enjoy or dislike messing with your blade.

During the autumn/winter/spring season I play 3-4 times a week, so the rubbers get replaced every 3-4 months when I start to feel the rubbers have lost grip beyond a certain threshold. Looping underspin becomes harder, slow loops seem to have less spin and are blocked more easily by opponents, underspin and side/underspin serves seem to lose bite. 

I play 2-3 times a week each time 3 to 4 hours, and my rubber has a lot of ball hitting spot showing in the middle already. It's still playable, but I'm just trying to find some distinct characteristic to determine when I should change it.

I'm too anal about the visual look of the rubber