So the main point behind the 5 min recommendation is that once the jam is over and people are playing and rating each other's games, they will want to get through as many games as possible - that means they are unlikely to spend more than 5 mins on your game unless it is really good! Even with a long jam, I'd focus on the first 5 mins above all else, polish that and make it as fun as you can, and then add more content if you've got time ☺️