The current fan theory is 7-8 years as that's the most time a studio has gone between making a movie. 2007 to 2014 when this was made is 7 years.
Elektra was 2005 and Marvel got the Daredevil rights back a couple years ago but has been sitting on them.
It was only 5 years after X3 that First Class came out (although Fox had done a Wolverine movie before that)