But it is not certain that a software fix will be the final solution.

If the FAA determines that there needs to be a hardware fix in addition to a software fix, that could further delay the return to service.

Boeing said it agreed with the FAA's decision to make additional changes.

"Boeing will not offer the 737 MAX for certification by the FAA until...it's safe return to service," said the company's statement.

But even if only a software fix is needed, it will likely be months after that fix is submitted in September before the plane is granted approval to fly again by the FAA.

That means the earliest Boeing and airlines can hope to have the plane flying again will be sometime close to the end of the year.

"I don't know if I'd say it's best case scenario, but that's what's likely," said Jeffrey Guzzetti, a former director of the FAA's accident investigation division.

The delay, at least in part, will likely stem from the fact that the FAA and Boeing will need to get near simultaneous approval from authorities around the world for the plane to fly again, Guzzetti said. More than 30 authorities from other countries have been meeting in the United States to discuss the certification process.

Getting FAA approval alone would be of limited benefit to Boeing since more than 80% of the nearly 400 grounded jets are owned by foreign carriers.

By the end of this year, Boeing will likely have about 400 built but undelivered Max jets in its inventory according to Cai von Rumohr, aerospace analyst with Cowen. But it won't be able to deliver those jets until after it fixes the planes already in the hands of airlines.

"Boeing's first priority will be to prepare the 381 planes in customers' fleets for service," he said in a note this week.

And it even once the deliveries start, it will take time to fix, test and deliver all those planes, said Jim Corridore, director of industrial equity research for CFRA Research. That means the delivery delay will likely stretch well into 2020. And Boeing doesn't get most of the cash from the sale of the plane until it's handed over to the customer.

"They're not going to clear out the supply in a month or two," he said.

"We're definitely looking at figures in the billions," he said. "It's impossible to quantify it completely at this point."

Even with all the problems for the company, shares of Boeing are up 13% for the year.

While firm orders for new commercial jets has virtually stopped, it has continued to deliver other types of jets and it has a huge backlog of past orders that will take years to work through.

"They're going to have to take on debt, eat into cash reserves," said Corridore. "As long as the plane comes back into service, the company will eventually be fine. And everyone expects that will happen. But whether it returns to normal in 2020 or not until 2021 is tough to say."