The mean half life of Cetirizine is 8.3 or so hrs. So that’s presumably where the feds came up with the mandatory 48 hr wait. On the other hand, the non-sedating antihistamines are pretty much unrestricted if they are tolerated and not taken daily. The other thing they really want to make sure of is that the airman doesn’t have any issues with blocked eustacian tubes or sinuses that could cause an in-flight issue.
I generally suggest either Claritin or Allegra, (both OTC in the US) but there are certainly other options