Not required to eat any sugar at all. Your body will actually produce its own glucose if and when needed through gluconeogenesis [1].
Why neglect one aspect of our bodies digestive energy systems for just gluconeogenesis. Wouldn't you be better off eating a balanced meal of complex carbohydrates and unsaturated fats. Our bodies have multiple pathways to producing energy, focusing on using only one is silly and not the right approach because it wasn't designed to be that way.
Just because our bodies can survive doing a particular thing in the absence of another, doesn't mean that thing we're absent of isn't required.