Depends where and what you want to do, where you want to live, whether you have kids/dependents, how much travel, etc. Also depends upon whether you want to deplete capital while in retirement or live on the income your capital earns.
My friends and I had this hypothetical discussion a few years ago and figured that if you live in Toronto, with eg: two kids still in University, and you want to be able to travel a few times a year, live in a decent neighbourhood, drive a decent car for as long as you can, have emergency funds for future assisted living/emergencies, boomerang kids, aging parents, grandchildren to spoil, with leftover to pass on when you kick the bucket - then including your house (which should be mortgage free) you need about $4 million - that was a round number just blurted out and agreed without much detail. That's to live a comfortable independent middle class retirement in Toronto leaving a nice inheritance for your kids or charity. YMMV. Obviously you can retire with a fraction of this depending upon your needs, location, etc. (Also - don't forget the hidden line items - like enough cash for hobbying for as long as you can get it up....)
Edit to add: The above is for a married couple retiring... much less needed if you're single.