As long as your reception is being held in a temperature controlled space, you can leave the cake on display for 3-4 hours. If the room is hot, or filled with sunlight, you should not leave the cake out. The frosting will melt, and your cake can become unstable.
For best results, remove the cake from refrigeration at least 2 hours before serving. Let it sit out in a cool/temperate room during that time. You must let the butter in the cake warm up so it will taste moist and amazing! As long as the cake has time to come to room temperature prior to serving, your guests will be able to enjoy the dessert at optimal flavor and texture!