If you extend your PCT out you will run the risk of desensitizing your system, which is counterproductive to what you are trying to do. Like anything else, if you continue a regiment of medication your body will adapt and become dependent on that medication. 4 weeks is the tried and true amount of time that it takes for your body to start producing its own natural testosterone again.

You want to speed recovery along, but you should be pretty much recovered at the end of the 4 week PCT cycle. Your body should be producing it's own natty, if that is not the case then something else may be wrong. For example, if your balls weren't ready to accept the signal to produce test then your Clomid would have been wasted. In which case you would need to blast HCG and then rerun your PCT.