“I can do this all day” is practically a catch phrase for Captain America in his solo movies and the Avengers movies. Said four times total, three more than his comic catchphrase “Avengers Assemble!”. It is always at an emotional height of the movie Except for the last one. The last one is more of a joke followed with the punch line, “Yea I know”. See below for the quotes.
The first time Steve Rogers says “I can do this all day” is before he is Captain America. He is weak and out muscled by a bully. Yet, Steve keeps standing his ground. Featured in Captain America: The First Avenger.
Cap uses the phrase again at the end of Captain America: The First Avenger when facing Red Skull.
The third time Steve uses the quotes “I can do this all day” is a bit more bittersweet considering the circumstances. He says it while battling Iron Man in Captain America: Civil War.
The last time Captain America is heard saying “I can do this all day” is in Avengers: Endgame. Which ironically said against himself, causing a funny reply for the other Cap.
Captain America’s Reply in Endgame to “I can do this all day”, which is “Yea I know”.