1) It's fiction! Just relax and enjoy it!
2) If we do hazard an estimation, it would be that Superman never stops inhaling even though exhaling. His Kryptonian physiology allows him to take in continuously more air through his nostrils, while expelling gale force winds from his mouth.
3) As Superman doesn't need to breathe, and only does so out of human mimicry, all of the air he take in resides in his lungs, being compressed over time, so we see as a simple draw of breath, is added to days, weeks, or months of air stored in his lungs
4) What we see being expelled isn't actual air, but force that closely resembles it. A short draw of breath, thanks his Kryptonian physiology is converted into a force that much resembles a hurricane's power, but isn't actually breath being expelled.
Come up with your own theory. All of Superman's powers defy our knowledge of physics, so there's really no right or wrong answer, just what appears to be more plausible.
I think there was a post here long ago debating the freezing breath in particular, saying that the only way he could produce air colder than his own body heat was a form of hypokinesis, opposite of pyrokinesis, creating coldness with his mind. Sort of an extension of how John Byrne had him using telekinesis to fly. Maybe that also applies to the force of wind he can generate.
Or maybe he simply inhales and stores more air than we know. Enough to let out a continuous gust for several seconds or minutes when needed.