It would seem the entire overall story of Star Fox 64, and also its reimagining, Star Fox Zero, spans about 10 years.

Going by the manuals and guidebooks, the story begins with Andross being banished to Venom by General Pepper after he unleashed a deadly weapon of some kind upon Corneria. Five years later, the original Star Fox team are sent to investigate strange happenings on Venom only for its leader, James McCloud, to be betrayed and killed by Pigma Dengar, which was also meant to be Andross' declaration of war upon Corneria and the Lylat System, and he and his new army he build up over time would spend the next five years attacking them, leading to General Pepper calling upon the new Star Fox team, which leads directly into the events of Star Fox 64.

Curiously, most of SF64's ancillary material leave it vague on how long it's been between James' death and the events of Star Fox 64, typically only saying that "several years" have passed, all except for Pigma's biography in the official Japanese Star Fox 64 guidebook, which makes it more clear that it's been five years since he betrayed James and Peppy. Beta Star Fox 64 images, as well as the lost developer dub discovered in the Nintendo Gigaleak from 2020, show that Peppy was originally meant to have a line on Titania where he says "This reminds me of 10 years ago!”, obviously referring to when Andross nearly eradicated Corneria with his bioweapon, which is what led to his banishment to Venom in the first place.

Star Fox Zero, whose story and lore is almost completely identical to Star Fox 64, fully verifies that it's been indeed five years since James' death and also overall 10 years since Andross was banished to Venom, both through its opening prologue, the Star Fox Zero - The Battle Begins animated short that was created to commemorate its release, and also the official Japanese Star Fox Zero website.