I think there are too many factors that have to be considered before anyone can give anything other than an educated guess. Location would be the biggest factor. Do you live in an area where a lot of pesticides are used? In an area where any other harmful substance are used? In an area where deceases or pest can thrive?
This is one of the questions that if you asked 10 different people, you could end up with 10 different responses.
As far as requeening...I would thing if you have a healthy hive and a good laying queen, you would want to keep her as long as you can. Let the bees decide when it's time for a new queen and let them make their own. I'm not opposed to requeening if needed, but to requeen "just because"...I , myself, couldn't do it.