RPG : Make a world. But a few enemies in it. Make a few buildings wich u can enter. Let him grind. Bon appétit.
Action Adventure FPS : Make a world that fits the gameplay. Every enemy u encounter shouldnt be too hard nor to easy. It should scale the gameplay and experience of the gamer (Like the Adam in Bioshock, Augments in Deus Ex etc). Multiple choices. More then one way to the end.
There were a couple of games you could finish in 5-6 hours in a particularly way. If you had teleporters in GTA4 or RDR. You could easily beat the games in 5 hours. If you run and gun everybody in Hitman you finished the game in 4 hours. Every FF game would be 5 hours without cutscenes grinding and go from a to b shit. Without tricks like driving to the mission, sidemissions, grinding and cutscenes most of the games would be 5-8 hours long.