Don't know if this will help, but I first visited Paris 50 years ago, and have been once or twice a year since then - sometimes for a few days, sometimes for a few weeks - without ever getting tired of it. As far as Egypt is concerned, I've only been twice - once to Cairo and the sites on the Nile, the other time to the Sinai peninsula and Red Sea; but would love to go back to the ancient sites, and to Alexandria which I've never visited. (But why do you want to know how many times you should go; just go!)
Financial and physical ability being as they are, these two are the only ones on my "must see" list, although I have considered Barbados. Sylvday, sounds like at least a couple of weeks in each would be good. My husband has rellies in London, Kent, Milano, Como & Zurich, but he only fancies Alaska brrrrr. It was also a dig at visitor numbers, can I just sit in the corner.