Assume that you want a 10% error , you have 20 v between 220 and 240 so the error is less than 2V , your voltage smoothener must ensure that the voltage between two semi periods doesn't drop more than 2V. So the response time is 10ms*20v/2v = 100ms. I assumed that the ac freq is 50 Hz so a semi period is 10 ms. The things get worst if you don't have a with exactly like that (2v ripple voltage) , if you have a better smoothener (let's say 0,2v ripple voltage) you will have a response time of 10* 100ms = 1 s, if you don't use a good one (let's say 5v ripple voltage) you will have an error of 20v/5v ->25%