Store credit doesn't expire, so you can use it whenever it is convenient for you and your vehicle.
Your credit acts as a payment method and can be applied during checkout to reduce the amount you owe on your order.
You can use your credit along with a discount code, as it is a payment method and not a promotional code.
If you want to check your credit balance, simply log into your account and click on "My Account" in the top right corner of the website to view your balance.