Now, I know perfectly well that the literal and unhelpful answer to the question is long enough to burn sufficient fuel that it is no longer overweight for landing. What I'd like to understand is whether it is possible or not to give an answer about what's typical.
I am surprised that after flying all the way from Moscow to Ireland this aircraft had enough fuel on board to be still overweight (even given that it still had to go all the way to the Dominican Republic):
An Azur Air Boeing 767-300, registration VP-BUX performing flight ZF-973 from Moscow Vnukovo (Russia) to La Romana (Dominican Republic), was enroute at FL320 about 90nm south of Shannon (Ireland) when the crew decided to divert to Shannon reporting all their lavatories were blocked. The aircraft dumped fuel and landed safely on Shannon's runway 24 about 90 minutes later.
Typically, how much of an airliner's flight will be in an overweight-for-landing condition?
Does it vary so much between aircraft types, distances, passenger/cargo weights and other conditions that it's not simply possible to say what is typical, or is it possible to say that the case above is (for example) unusual?