You did the wrong math: you didn't account for air that gets sucked through the filter more than once. So that random filter is generally assumed to be about an hour to filter all the air in that room once. And that is assuming there is reasonable air circulation over the whole room - often what happens is the filter sits in a corner and filters that corner well and doesn't filter the rest of the room at all.

More filters in the room, and different sized filters both exist.