What's the speed of convergence to the potential GDP? (Empirical estimates)
First of all the speed is not constant, it declines as one gets closer (at least according to the text book predictions).

A classic paper on among others this issue is the one by Mankiw, Romer and Weil in the 1992 QJE "A Contribution to the Empirics of Economic Growth" They find convergence rates of about 1 to 2%. The paper is cited 16,000 times in Google scholar, so no doubt you'll find some material there too. I'm no macro-economist, but it seems an area of active research.