The current official Early Access listing describes roughly 3–6 hours of gameplay and three gyms. That estimate applies to the current Littleroot-to-Mauville build, while the older island demo is a separate historical release with no current completion estimate in the available source material.
WALKTHROUGH / how long is the pokemon gamma emerald demo
The current estimate
The official page gives 3–6 hours as the current gameplay scope. It also identifies three gyms and the route from Littleroot Town to Mauville City.
This is an estimate for available Early Access content, not a timed guarantee. Exploration, battles, optional systems, and troubleshooting can change how long a player spends in the build.
Demo and Early Access are different
The old demo takes place on a mysterious island between Hoenn and Sinnoh and includes its own non-canon story, puzzles, Team Magma and mercenaries, legendary Pokémon, and original assets. It is marked obsolete and unsupported.
The current Early Access build uses a new engine and follows the opening Hoenn route. A video titled Demo may therefore be describing a different experience from the current version.
What can extend a session
Followers, day/night encounters, berry growth, hidden items, trainer rematches, the daycare, Game Corner, PC storage, and P2P trading can all add activity beyond the direct gym route. The current page also lists battle abilities, held items, weather, status, and modern physical/special rules.
These features explain why a play session can vary, but the supplied evidence does not support a larger exact hour count. Keep the official 3–6 hour estimate at the top of the answer.
What is not included yet
The area beyond Mauville remains in development, and future roadmap items such as more Pokémon, a National Dex, and localization are not part of the current time estimate. A final full-game length is待确认.
Pokémon Gamma Emerald is a free, non-commercial fan project by UndreamedPanic. It is not affiliated with Nintendo, Game Freak, Creatures, or The Pokémon Company.
Current Early Access facts are version-labeled. Historical Demo details, community reports, and unverified routes are kept separate and marked 待确认 where necessary.