Increase the length of the z-buffer I am using the examples already given here, i.e.
Convert depth/z value to RGBA

The problem I am facing here is to increase the depth. If one sees clearly a blue pattern starts from a close distance of the camera. I need to push it further. Please 

I have tried changing the values of the nodes but it is not working.
Your setup is fine. You take depth Values (a number of some value per pixel representable as an grey scale image). And dependent on this number you chose a color from the HSL spectrum. 
So you just need to scale your depth value right to controle for which depth wich color should apere.