Prince Charles is first in line for the British throne, and therefore, when his time comes he should take ascend to it. Not only is it expected, but it will give his son, Prince William more time to help raise his young family before he becomes the King after Charles' passing.
Prince Charles should take the throne if he is so entitled. I suppose he could give it up to his son, William, who is younger and probably fitter to be the King. However, I am not sure whether this is even a possibility based on the Royal Law or whatever it is that they have to regulate this kind of issue.
Prince Charles hasn't put forth the effort to become the type of monarch his mother is. He has come across as a whiny, indignant prince who would stamp his foot in anger rather than diplomatically diffuse any kind of situation. He has attempted to rule via backdoor politics and gotten caught at it. He must become the representative of the entire country of Great Britain. He doesn't have what it takes.
This is just my opinion but I suspect that one of the reasons why Queen Elizabeth hasn't stepped down from the throne of England has to do with the fact that Charles isn't too popular in the public eye. Maybe this is just an opinion of someone on the outside, but there are those who adored Diana and know that one of the reasons why their marriage fell apart was the fact that Charles loved another whom he eventually married. It may seem like all of this is ancient history but there are those who still remember it all too well. So, maybe Charles should do the honorable thing to help give him a better view in the public and advocate to the next heir to the throne his and Diana's son William, along with his wife Kate. They are quite popular to the people of Great Britain and no doubt could help bring the Monarchy into the 21st century.