This is the relevant Guidance issued by DOT. It applies here because this was a departure from the US. While there is no time limit on when a mistake becomes not a mistake, the more time that goes by, the more expenses a passenger is likely to have incurred:
[The carrier] reimburses all consumers who purchased a mistaken fare ticket for any reasonable, actual, and verifiable out-of-pocket expenses that were made in reliance upon the ticket purchase, in addition to refunding the purchase price of the ticket. These expenses include, but are not limited to, non-refundable hotel reservations, destination tour packages or activities, cancellation fees for non-refundable connecting air travel and visa or other international travel fees. The airline may ask the consumer requesting out-of-pocket expenses to provide evidence (i.e. receipts or proof of cancellations) of actual costs incurred by the consumer. In essence, the airline or seller of air transportation is required to make the consumer “whole” by restoring the consumer to the position he or she was in prior to the purchase of the mistaken fare.