Duration is relevent for an IR signal that has a short frame that must be repeated many times to make a signal that takes enough time.

Air conditioner remotes often have a super long frame, too long for an ordinary learning remote to learn it. That still might not represent a lot of time. I mean "long" in terms of the Pronto Hex string that would be needed to represent the signal. If the individual IR bursts are fairly short, there can be a lot of IR bursts without taking a lot of total time.

If you post some Pronto Hex from learning attempts, I can give a better estimate of what is happening and whether there might be a work around.