The time spent in the air, around 306 hours to be precise, would allow you to fly around the world seven times!
The survey reveals that people living in London and the South East will spend the most time in the air.
The team behind the online travel agency www.sunshine.co.uk conducted the research as part of an ongoing study into the travel habits of Britons.
"The travelling time to and from our holiday destinations soon starts to add up, particularly after a few long-haul flights," said managing director, Chris Clarkson.
"When we see that we could spend as many as 12 entire days on a plane in our adult lifetime, it just goes to show how important it is to put some thought and research into your bookings; making sure you're confident in the airline's ability to deliver a comfortable experience.
"We'd love to do further research into this topic, to see what people do to entertain themselves during all of these hours in the air.
"When a personal entertainment screen is lacking, it can be quite boring on a flight. However, there's plenty that can be done to pass the time and make the journey more enjoyable, like playing games and reading a good book!"
A total of 2,486 people from around the UK, all of whom were aged 22 and over, took part in the study and answered questions about their holidays and travel experience; particularly in relation to air travel.
Initially, everyone taking part was asked to estimate how long they had spent on flights as a passenger over the past five years, including travel to and from holiday destinations (stating '0 hours' if they had not flown at all).
The results revealed that the average UK adult spends 34 hours in the air on flights during the average five year period (averaging 6.8 hours per year).
Everyone taking part was asked how much of their adult lifetime they thought they would 'holiday regularly' for; revealing that the majority felt that they would go away regularly until they were '63' years old.
This is just below the average retirement age in the UK.
When looking at a time span of 45 years from 18 to 63, the average UK adult will therefore spend at least 306 hours as a passenger on flights in their adult lifetime.
Also, looking back over the past five years, those who had flown were asked how many hours of the flights they had spent sleeping or napping, revealing an average answer of 10 out of the 34 hours they were in the air for during a five year period; or 90 hours sleeping mid-flight during their adult lifetime.