If I remember correctly official demos of some of the Pokémon games have been released, with the player typically playing up to the point where they'd next challenge the first Gym. However, that's for a project that might already have some of the later content already in the game, just blocked off, and that will likely be out by year's end, which may not be true of a fangame project. I'd say that it's a good first demo if you can get to the second or third badge, depending on how spaced out each badge is. Basically, at least a quarter of the game's main story, whatever that ends up being for badge count.