Tell the truth. I don't know the answer. But for

2011 Optima, the answer probably is "Never". Most of the "dusts" you would find in a throttle body is the carbon deposite caused by the EGR(Exhaust gas re-circulation) system. The EGR is for reducing the NOx emission. It has been around for a very long time. But the 2011 Kia Optima does not have an EGR system. Instead, the engine used variable valve timings both on intake and exhaust ports to acheive the same result. Ideally, the throttle should never see carbon deposite on it.