If they're mothballed (skin covered and various systems pickled), then about a month of work each.
If they're not mothballed and they do weekly or bi-weekly maintenance on each plane like they're supposed to, then an inspection, and then they're ready.
So the worst case is a month, and the best case is an inspection plus 30 minutes.
Also, note that in the mdoern era, a lot of avionics require periodic updates, which also have to be performed before flight.
But if I was the FAA, I'd be requiring crew-only acceptance flights before any revenue flights. You never know where those pesky wasps built a nest, or if those AOC sensors corroded, etc.
A 757 crashed in 1996 when it sat for three weeks and gave mud dauber wasps time to build a nest in the pitot-static tube that fed airspeed information to the autopilot:
To be left unattended with the intention of flying them again, mothball/pickling is needed. (Tape the outside including sensor ports, plug the engine nacelles, use the right hydraulic or motor fluids, disconnect batteries, etc.)
If you literally just park a 737 somewhere for months, especially a sandy desert, and walk away, then you might have to do millions of dollars of maintenance on the plane to be sure it's airworthy. Like pulling the engines and dealing with corrosion throughout the airplane.
I have a small airplane example. The danger of parking an airplane for an extended period of time, even in a hangar, is that if mice create a nest in the fuselage and urinate there, the aluminum corrosion can cause structural damage. ie. that plane will never fly again.
All those King Airs, Cessna 310s, etc. you see hangared are prone to that. So clean your airplane regularly!
I was just envisioning what this storage airport would look like in 10 years if the airline industry NEVER picks back up to its previous levels. Trillions of dollars worth of airplanes rotting in the sun???
The point is those updates may have expired, and it's required to have updated avionics data before flight, not if OTA updates are available or not.
You're supposed to have "all available data" with you during flight, but for local VFR it's not as big an issue as for IFR or Parts 121/135 operations.
In Canada, they rename the position fixes to catch you. Messed up, but very effective! Expensive too, as I was buying plates for just one flight during the update period.
(I say it's messed up because pro pilots in the US can/are expected to memorize fixes and frequencies, but one of those is not possible in Canada.
I was listening to 2 Aloha Airlines pilots talking once, and the captain gave his copilot shit for not memorizing the frequencies they have to use every day.)
