How often should a high quality mattress be replaced?
I’m interested in your personal experience, not an answer from a sales person. Our mattress is in what I believe to be good condition without any problems, but am wondering what the “life” is of an average mattress. And, any thoughts on the box spring would be helpful too. Many thanks!

I would say about 5 or 6 years.
I would say if you flip the mattress occasionally, it should last for a long while. However, if you are not getting restful sleep, your back starts hurting, things like that, it may be time to consider getting a new one. I have had mattresses that have lasted years with no problem.
As far as the box spring is concerned, I have always just replaced it at the same time. However, after recently getting a mattress, I was told (by a salesperson) that if the box spring is in good condition you can just keep it to use with the new mattress. If the springs are worn, etc., you may want to just get a new one. You do not have to get an expensive one however! Basically if a box spring is constructed properly, it does not have to cost a lot of money.
I know that in my situation I had my mattress and box spring for a long time so I just got new ones. I had gotten them as a “package” deal. I just bought a house and wanted to “go all out” so to speak, lol. However, down the road, should I need a new mattress, I may just replace it and hold on to the box spring if it’s in good shape.
Anything less than 10 years is nonsense, especially if the mattress is high quality, and a box spring is so inexpensive that you should pick one up at the time you buy a new mattress, if only for the heck of it.
It’s really a matter of how it works for you.
If you are getting restful sleep and wake up without back issues, then you’re good to go, regardless of how old the mattress is.