It's a ionizzzzzer!

Long-acting ionizer that create per unit time per unit volume of air ion pairs $n_i$ = $3.5 \times 10^{9} cm^ {-3} s^{ -1}$ , has been turned off. Assuming that the only process tending to reduce the number of ions in the air is their recombination coefficient r = $1.67 \times 10 ^{-6} cm^{3} /s$ , find how long after turning off the ionizer, ion concentration decreased $2.0$ times.