As for “he won’t be king for long” - probably 20 years, I’d say. His parents both lived into their 90s and he is a health conscious non smoking vegetarian!
The previous queen had an unusual long reign because she represented a particular turning point in the increase in life expectancy. Her father, coming from less health conscious times and not having access to modern medical care, died in his 50s, and this led to her becoming queen in her mid-20s; she then, of course, became part of the generation that had access to the best of the modern’s world’s healthcare as she grew older, and therefore lived well into her 90s.
From Charles onwards, monarchs will probably occupy the throne for around 30 years each.
If they do bring in a retirement system (which I personally thinks makes sense), a sort of 40yo-70yo schedule for the monarch would make sense IMO; start the job at around 40 when you and your spouse are probably not having any more babies, and then retire around 70 when it’s time to start slowing down and give someone else the job.
I’d like to see Charles retiring about ten years from now, ideally, but I bet he’ll cling on as long as possible!