I have never understood how much filling to use between each layer of cake...I do it differently every time. What is the general amount on average to use per layer?

It depends on what you're using and how you like to do it. I've seen some cakes where there's more filling than cake, but that's not my preference. Depends on what you like...

The Cake Bible has a chart that gives amounts for buttercream fillings, and I find it to be completely accurate based on how I fill my cakes. If you like a really thick filling it would probably be too little for you, though.

Hope this link help.
http://www.ladycakes.com/filling_chart.htm

Thanks everyone! I have been wondering if there is a filling chart out there myself!