Two minutes, twice a day, with a fluoride toothpaste. Then do not rinse — spit, and leave the fluoride on the teeth. Rinsing washes away the part that was doing the work.
Cleaning between the teeth matters more than most people believe, because that is where decay and gum disease usually start. Interdental brushes beat floss for most adults, simply because people actually use them.
And timing beats frequency where sugar is concerned. A biscuit with lunch is one acid attack; the same biscuit broken up across an afternoon is five.