converting a muffin recipe to a cake recipe
Most of the bake times that recipes give you are very general ideas of how long you should bake something. This is why most of them are given in ranges rather than in specific values. (20-25 minutes instead of specifically saying 21 minutes)

The reason for this is because there are a lot of variables when it comes to baking including the thickness and material of your pan and the type of oven you have.

The best thing to do when you are trying to bake batter for muffin/cupcake in a cake pan is to start out with the initial time the recipe gives you and then there after, check it periodically with a toothpick in the center. If the batter sticks to the toothpick then it is not done. You can check it every 5 minutes to start off but as the "cake" start to stick less to the toothpick, check more often.