My question is how often and for what size parts do you use your airbrush as opposed to brush painting?
--> It depends. Some people airbrush everything and others paintbrush small stuff.
What do you do with your left over mixed airbrush paint?...I bough extra mixing bottles with covers to store leftover mixed paint since I need to learn how much is enough for a particular model.
--> You can reuse mixed/thinned paint. Make sure the cap is airtight. Otherwise, the paint will go wrong.
Is 2 parts paint to 1 part thinner for acrylic the standard ratio?
--> Depends on the brand. For Tamiya, I use a 1:1 ratio.