I'm glad I wasn't the only one to start looking short films up! Add in Rec at 78 minutes and Before Sunset at 80
I wonder if there is an actual cut off? Presumably anything under 70 minutes maybe?
Actually according to Wikipedia:
A feature film is a film with a running time long enough to be considered the principal or sole film to fill a program. The notion of how long this should be has varied according to time and place. According to the Academy of Motion Picture Arts and Sciences, American Film Institute, and British Film Institute, a feature film runs for 40 minutes or longer, while the Screen Actors Guild states that it is 80 minutes or longer.
The majority of feature films are between 70 and 210 minutes long
40 minutes seems a bit on the low side however, and that would mean every modern episode of Who has been feature length!