Having been operated long enough, the ionizer producing nt = 3.5.109 cm-3 ∙ s-1 of ion pairs per unit volume of air per unit time was switched off. Assuming that the only process tending to reduce the number of ions in air is their recombination with coefficient r = 1.67.10-8 cm2/s, find how soon after the ionizer's switching off the ion concentration decreases η = 2.0 times.