How Long Has [SPOILER] Been A Skrull In The MCU?

Warning: This article contains spoilers for Secret Invasion.Secret Invasion's opening scene casts doubt on Everett Ross' MCU history with the reveal that a Skrull has been impersonating him. The main appeal of Marvel's Secret Invasion storyline is the sense of distrust caused by the Skrulls' shapeshifting abilities. In the comics, Skrulls infiltrate Earth and replace several A-list superheroes, including Hawkeye, Spider-Woman, Elektra, and Hank Pym. With no way to identify them, the Skrulls are able to destabilize all superhero teams and extend their reach over the world.
Although Marvel Studios gives Secret Invasion a Disney+ series of its own, there's little time to adapt the storyiline into a franchise-wide event that involves the Avengers and other big-name MCU characters. There's also not enough space for major retcons, as the possible reveal that an Avenger has been a Skrull in disguise all along would lack the appropriate setup. Still, the MCU's Secret Invasion is expected to feature more than a few Skrulls shapeshifting into known Marvel characters before Nick Fury and his allies get a chance to stop them. Secret Invasion episode 1 already started delivering such surprises with Everett Ross' Skrull twist.
Agent Ross Is A Skrull In Secret Invasion Episode 1 Secret Invasion episode 1 delivers the shocking reveal that Agent Everett Ross, who falls to his death while being chased, was a Skrull impostor who wanted to keep Agent Prescod's invasion theory from going public. Unlike Talos, most Skrulls intend to use humans as pawns in their plan to take over Earth, and deception is the perfect tool for a species whose superpower is shapeshifting. Since Secret Invasion also reveals that Skrulls are able to steal identities by copying memories, finding out which human character is a Skrull in the MCU has become even more difficult. In the case of Everett Ross, however, some clues suggest that he has only been replaced recently.
Agent Ross Probably Hasn't Been A Skrull For Long In The MCU The Skrull that poses as Everett Ross at the beginning of Secret Invasion episode 1 meets with Agent Prescod in modern-day Moscow. However, Valentina Allegra de Fontaine arrested Agent Ross for treason against the United States in Black Panther: Wakanda Forever. Before he was sent to trial, Okoye intercepted the FBI's vehicle and freed him. Although the Wakandans got Agent Ross out of immediate trouble with the US government, Ross likely needs to stay out of American territory while he proves his innocence. Hence, the real Agent Ross likely remains in Wakanda, giving the Skrulls a chance to impersonate him in his absence.