The theoretical treatment of alpha-decay by Gamow is revisited by investigating the quantum time scales in tunneling. The time spent by an alpha-particle in front of the barrier and traversing it before escape is evaluated using microscopic alpha-nucleus potentials. The half-life of a nucleus is shown to correspond to the time spent by the alpha knocking in front of the barrier. Calculations for medium and super heavy nuclei show that from a multitude of available tunneling time definitions, the transmission dwell time gives the bulk of the lifetime of the decaying state, in most cases.