Every so often I hear someone complaining about how short a video game is and I always break it down the same way. A movie is about $12 (pre-upgrades) for two hours of content. Why do we then have a different set of standards for video games? I love ending a game and wanting more. That's the best criticism anyway isn't it. "I didn't want this to end."
John Waters puts a 90 minute cap on all of his movies and I think that's a great way to ensure that the film your creating is boiled down to essential parts.