Comment by searedsteak

It is a requirement [1] to land with 45 minutes of fuel remaining, if the pilots go under that, it is considered an incident. As soon as estimated landing fuel goes under the limit, the flight needs to declare an emergency (as was done in this case).

[1]: https://www.ecfr.gov/current/title-14/chapter-I/subchapter-F... is the US rule, EASA has a similar rule.