When I use a "cake" recipe but want to make cupcakes is there a rule of thumb to follow in regards to changing the bake time? 

Also...I've seen a few recipes that the poster says "if you want to make this moister you can add sour cream...my question is...how much sour cream? 

Any advise would be appreciated!

There isn't any real rule as far as baking time. Every recipe is different and every oven is different. You have to find what works for you. Personally, my cupcakes take about 12-20 minutes depending on the recipe. I can usually smell when it is done but just check for doneness. As for the sour cream, not sure. I don't add it to mine. I have subbed greek yogurt for half the milk in a recipe before.