That really depends on how you define epic. The term epic has been used in advertising to mean epic scope, epic length and many other things.
Some of the earliest films of epic length are:
The Photo-Drama of Creation (1914)
Les Vampires (1915)
Arshin Mal-Alan (1916)
The Protege of Satan (1917)
These films are 6 to 8 hours long and were "epic" in length for their time.
Cinematon (2014) on the other hand began filming in 1978 and took 36 years to complete. The completed film is 11,460 minutes long and takes 8 days (without breaks) to watch. Some would consider it the first epic film making experience as it took the creator nearly his entire adult life to film.