My entire life, ever since 1970, I think I have fasted at least 12 hours per day.  That's the time between the end of dinner (7pm latest) and the start of breakfast (7am earliest). Generally I've eaten dinner at 4-5pm though.

And I am overweight with metabolic syndrome.

For me to get any kind of measurable improvement I have to fast for at least 16, maybe 18 hours per day, something like breakfast at 11am and dinner at 4pm.