Every time you buy food or drink, you receive credits to play. Most of the time your playing will be free. If you just sit there and milk your food, it will ask you if you want to buy credits. They have a secret algorithm to assure on average to make a gross profit per hour per guest, and that's what determines if/when a guest will be asked to buy more time or buy more food or drink.
But for the most part, all entertainment is usually free.