Fewer than two-thirds of holiday bookings are for seven or 14 nights, a fall of 11% since 2009, according to Jet2.
The operator, which claims to have pioneered flexible holiday durations 10 years ago, said one in four holidays are now for between 8 and 13 nights.
The number of bookings between one and six nights has also risen four percentage points to 11% over the past 10 years, said Jet2.
It claimed the trend towards non-standard durations was most evident at East Midlands Airport, where there has been a 21 percentage point drop in seven and 14-night bookings, which are down to 66% compared with 87% in 2009.
There has also been a marked drop at Newcastle Airport, down from 82% in 2009 to 66% this year.
At Manchester Airport, the company’s biggest base, there has been a fall from 73% in 2009 to 60%.
The pattern is noticeable in all age groups, except families.
Jet2 CEO Steve Heapy said: “There was a time when holidaymakers would have to book a seven or 14-night holiday by default, but we knew that they wanted more flexibility and we were the company that responded to that.
“Instead of telling customers how long they should be holidaying for, we wanted them to be in complete control, so we pioneered the concept of flexible durations, operating multiple weekly flights and even daily flights, so that customers could be more flexible and have more choice.”