How long is half life 2

Half-Life 2's game-play length depends mainly on whether or not you mind watching conversations in-game or running ahead while NPCs are still delivering their lines. It is suggested that you take in the story.

Without paying attention to story, 4-5 hours
Paying attention to story, 5-10 hours depending on the player and their devotion to Achievements/Trophies/Steamworks Medals (ie. see Lambda Locator achievement)

(Times are roughly estimated as I've played through it both quickly and taking my time)