https://www.speedrun.com/games#platform=GBA&orderby=mostruns&unofficial=offThe WORLD RECORD speedun for GS is 3+ hours
It isn't just the longest Pokemon game (including Mystery Dungeon even)
Not even Metroid, Zelda and Final Fantasy on the same platform can compete, and no other game is over 3 hours for the *WORLD RECORD* (meaning you won't even be close to doing this time even if you tried instead of just playing normally)
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3h 13m Pokémon Gold/Silver
3h 10m Pokémon Crystal
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2h 30m Pokémon Emerald
2h 29m Pokémon Mystery Dungeon: Red/Blue Rescue Team
2h 3m Pokémon FireRed/LeafGreen
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1h 57m Pokémon Ruby/Sapphire
1h 54m Final Fantasy IV
1h 54m Pokémon Yellow
1h 45m Pokémon Red/Blue
1h 38m Final Fantasy Legend II
1h 38m The Legend of Zelda: Oracle of Seasons
1h 37m The Legend of Zelda: The Minish Cap
1h 32m The Legend of Zelda: Oracle of Ages
1h 22m Final Fantasy
1h 18m Rayman
1h 11m Metroid Fusion
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59m Mario Kart: Super Circuit
51m The Legend of Zelda: Link's Awakening DX
46m Pokémon Trading Card Game
43m Mario vs. Donkey Kong
40m Dragon Ball: Advanced Adventure
39m Pac-Man
38m Final Fantasy VI
38m Kirby: Nightmare in Dream Land
37m Metroid: Zero Mission
33m Tony Hawk's Underground
33m Tony Hawk's Pro Skater 4
29m Castlevania: Harmony of Dissonance
28m The Legend of Zelda
27m Kirby & The Amazing Mirror
18m Zelda II: The Adventure of Link
13m Castlevania: Aria of Sorrow
11m Kuru Kuru Kururin
10m Super Mario Advance 2: Super Mario World
9m Metroid (NES)
9m Dr. Mario (NES)
9m Pac-Man Arrangement
7m Super Mario Bros.: The Lost Levels
5m Mortal Kombat II
4m Ultimate Mortal Kombat 3
4m Doom (GBA)
4m Super Mario Bros.
4m Tetris (Game Boy)
3m Tony Hawk's Pro Skater 3
3m Strider (NES)
1m Tetris DX
1m Super Monkey Ball Jr.