Controversial Boeing jets taken out of service since two deadly crashes will need up to 150 hours of servicing before they can fly again.
In March 737 MAX planes were grounded worldwide after two separate crashes in less than six months claimed the lives of 346 people.
If regulators approve the jets for flight, each aircraft will likely need between 100 to 150 hours of preparation before they're safe to fly, officials from the three US airlines that operate the MAX told Reuters.
The preparations were discussed at a meeting between Boeing and MAX customers in Miami earlier this week, and listed fluid changes, engine checks and new software as essential.
The estimated time frame does not include pilot training.
With 34 jets Southwest was the hardest hit by the grounding, followed by American Airlines with 24 and United with 14.
No major British airlines use the 737 MAX.
Airline officials stressed that the planes would only be removed from storage once regulators approve Boeing's software update.
It is hoped the patch can fix a system called MCAS that played a role in both crashes.
Boeing has yet to formally submit the fix to the FAA.
US Federal Aviation Administration Chief Dan Elwell said there was no time table to approve the plane for flight.
As well as making sure the plane is up to scratch, pilots may need additional 737 MAX training.
Regulators are debating whether pilots should test crash scenarios in a simulator, which would cost airlines more time and money than Boeing's proposed computer-based training.
Boeing has pushed for a mandatory audio course that can be completed at home in an hour.
So far U.S. airlines have canceled MAX flights into July and August, taking a hit to revenues during the busy summer travel season.
Southwest has parked its MAX jets in hangars out in the California desert, while American has parked 14 of its 24 jets in Tulsa, Oklahoma.
