I don't know what you are smoking, but I hope you are smoking it when I play against you.
Every sub's air time is different. To prove it to myself, I just ran a test with the KM SS5 and the IJN SS5. KM stays under for 217 seconds, the IJN for 229 seconds. Not a lot of difference, but the IJN is under for 5.5% longer. Subs of the same level will have similar air times.
For recovery, the planesman is the most important aspect, but different subs have different recovery times.
Traditionally, the slower the sub, the longer the air time. So, for everything I know, the IJN subs stay under the longest. I am sure there are posts out there (maybe on the old website) which tell you exact numbers -- I don't have the time to run them all down.
Which brings up a fun point about IJN SS5's with good crews. They are the best for a HA decoy fleet. You first launch 3 TBs to scout the other team, then you wait for them to get near, go under and run in OH as far as you can. The TBs fool the other team into thinking a CV is in the area, and the long underwater time is hated if they don't have ASW. One game, I kept the room open for like 30 minutes because they couldn't find me! Another one, my TBs found the BB and CV in the room and me and another sub killed them.