Good answer right here.
I would aim for 5 seconds.

For what it's worth, I would add....
I think that what works well is the photographer need to take complete command of the session and be very vocal as to what is needed from the fish holder. A vocal play-by-play from the photographer.
If the fish holder is a newb, then a little rehearsal is in order while the fish is being revived. Now is also the time for positioning in regards to background and lighting.

"OK, lift...turn the bottom out....hold that....got it."

The pics that I like are when the fish is supported by the "pectoral bridge" (not the belly) and gently held around the wrist of the tail.
With a feisty fish sometimes that is very hard to achieve.

So, I said 5 seconds, but in reality I have done that up to three times with plenty of revival time in between.