Elizabeth oversaw remarkable changes in the British Empire, as it evolved into a Commonwealth.
It took Charles a long time to grow into the role, but he is actually pretty well-suited to be King at this point, and Camilla has turned out to be a decent person who will make a good Queen Consort.
I would guess he will stay in the role for 5-10 years, and will streamline the position and reduce the pageantry, which will be a loss for fun TV events, but probably for the best. He's done a lot of good work as the Prince of Wales.
The Queen was pretty amazing, but really might have better abdicated about 10-15 years ago.