Can search Ad $100 prompt code be used for multiple bills?

I just got the $100 free credit and added to my account. Does that $100 only apply to the first billing order? (like if I run a $25 campaign, it will be free and the free credit will be expired). Or can I use it for multiple times (Like I run two campaigns, with $50 budget each, and these two will both be free)? How to check the remaining balance?