As with all microwave cakes, you will need to experiment to find exactly how long to microwave the cake in your microwave and using your particular mug. I use a 12-oz mug in this recipe. Try to stick with a similar size mug. Using a much larger mug (such as a 16-oz mug) could dry out the cake. A smaller mug could result in the cake spilling out of it while in the microwave. As for how long to microwave this cake, I would start with 30 seconds, then check to see how it looks. Your goal is to keep it soft and gooey, but not liquid. It's definitely better to err on the side of microwaving too little than too much. You can always keep microwaving in 10-second increments, but you can't un-microwave a dry cake!