I don't think it's that simple. There are too many variables. It's probably different for each person and depends on a host of health-related things. It's probably also different for the same person at different stages of life.

Wouldn't it be nice if there was a simple number or rule or formula? We're not that lucky. You have to pay attention, preferably with the help of sensible people who know you and have an interest in your continued well being. And yeah, that can be the luck of the draw, too.