Airlines must refund checked bag fees if the bag arrives late, the Department of Transportation said Tuesday.

But how late is late? That's yet to be determined. And it could take DOT deep into 2017 to figure it out.

The stakes are high: Airlines collected $3.8 billion in bag fees last year and reported mishandling 2.58 bags for every 1,000 passengers.

“If you’re paying for the service of baggage delivery, you should expect to receive that service and that baggage in a timely manner or receive a refund,” Transportation Secretary Anthony Foxx.

Before DOT can determine how much of a delay triggers a refund and how travelers will get their money back, it must collect comments from stakeholders, such as airlines and travelers.

Congress told the department to complete the rule by July 2017 and also set some parameters in a Federal Aviation Administration law adopted in July. The law says airlines must refund checked bag fees if luggage arrives more than 12 hours after a domestic flight or 15 hours after an international flight.

The transportation secretary could extend the deadlines to 18 hours for domestic flights and 30 hours for international flights.

The parameters are a compromise. The House version called for refunds after a 24-hour delay. The Senate version called for refunds six hours after a domestic flight and 12 hours for international.

Rep. Michael Capuano, D-Mass., urged a 12-hour deadline in the Transportation Committee.

“Because the DOT is in the process of soliciting comments about this baggage fee proposal, he’s not surprised that a time frame isn’t established yet,” spokeswoman Alison Mills said. “As this process moves forward, he will be encouraging the DOT to act with the best interests of travelers in mind.”

Airline partnerships and other logistical considerations complicate the process. DOT must determine, for example, how to treat luggage handled by two different airlines when passengers travel on connection flights. The department will also weigh how the airline should refund the fee. An existing regulation calls for refunds on credit-card purchases to be refunded through the card within seven days, and through other means within 20 days.