fifteen years
Note from 'Visits Sheffield' chapter from 'Memoirs Of My Life And Writing' by Gibbon

Mr. Buffon, from our disregard of the possibility of death within the four and twenty hours, concludes that a chance, which falls below or rises above ten thousand to one, will never affect the hopes or fears of a reasonable man. The fact is true, but our courage is the effect of thoughtlessness, rather than of reflection. If a public lottery were drawn for the choice of an immediate victim, and if our name were inscribed on one of the ten thousand tickets, should we be perfectly easy !