Tunneling ionization is a basic process of strong-field atomic physics. Revealing its time-resolved dynamics is one of the goals of attosecond science. Here, we show that after tunneling, a finite response time (about 100 attoseconds) is needed for the electronic state to evolve into an ionized state. We construct a semiclassical model with a compact expression to describe this response time. With this expression, a simple Coulomb-calibrated mapping relation between time and observables is obtained. Comparisons with experiments give direct evidence for our theory. Our work uncovers the transient response process around tunnel exit and provides a simple tool for quantitatively explaining and predicting experimental phenomena in attosecond measurements.

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