The credits have an expiration date though. And while I'm not aware of how you can find out when these expire, I am aware of how to find out how much you have available. The How to Check Your Account for Digital Music Credit page has a Check your balance button. Click on the button and a popup shows your credit.
Here's what the popup window looks like. Clearly, I've spent everything I had, but you'll see a balance under Amazon MP3 if you didn't.
Start collecting those Pantry credit next. With those, you can only use $5 per $50+ order though so don't overdue it if you aren't planning on buying that much for the pantry from them.
*Read my Disclosure