This is a tough question! When airlines have price drops on various routes, they tend to last at least for a couple of days - if not longer. That being said, some unannounced flash sales will be shorter. As you might have already noticed, we try and predict how long these fares will last in the 'Book by' section of our email alerts:
This estimate is based on my experience (from staring at flights from the UK every day for a few years!), but is obviously not fool-proof. In reality, no one can predict exactly how long a deal will last, so any deal can expire at any time - sometimes even within an hour or two of me sending out an alert.
The rule generally goes as such: the better the deal, the quicker you should try and make a decision as the really good ones are, understandably, those that are more likely to sell out. Airlines drop their fares when the demand on a particular route is falling short of its forecast and, as soon as seats fill up to meet their expectations/projections, they'll usually raise prices right back up.
With Error/Mistake Fares in particular, I recommend booking as quickly as possible as they'll often last less than a day.
If you're checking a deal and following the 'How to book' instructions but the fares on Momondo/Kayak or on the linked website are much higher than those I've listed, this sadly means it's most likely that the deal is already gone.
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If you still have more questions, feel free to drop me a message on [email protected] and I’ll be happy to help.