That depends on how long you want your game to be. What were you imagining?
I've played some short games and they can be anywhere from 15 minutes (a mystery, puzzle, horror game, or any game that makes a simple, interesting point. Or just a short, enjoyable experience) to maybe a couple of hours, or longer.
A medium length game? Hm... guess it would depend on how much you want to flesh out your story and expand your world. For me, I love characters and story and music, the mechanics/type of game come last. For how many hours? Hmm... well, maybe, say, 10 - 20 hours? I'm purely guessing here based on my intuition, feel free to take this with a grain of salt.
For a long game... A great example of a fully fleshed out long game with an expansive world and engaging story and characters is Final Fantasy VI (or III if you're the technical type). That game can be as long as you want (how long you want to play/grind/experience everything).
*Long game: I guess anywhere from around 25 hours, to 80 hours and beyond.
Like I said, take my advice as you will. But I hope I helped you.
ps: something to keep in mind: the length of a game all depends on what kind of game it is (technically) and how you want people to experience it. What I mean is,
do you want there to be lots of backstory? Character interaction with a fair amount of dialogue? Is the story dark, in that there's a villain and it's going to take time to take 'em down? Long game.
Is the story a puzzle game where you want your players to enjoy a nice hour or two of challenges and fun? Short game.
Do you want a game that leaves the player happier after having finished it, in that it wasn't so long they got discouraged or bored of repetition? Maybe a game where there's a focus on one specific thing like music, story, battles? Medium game.
