The application for certification (and fee) ytou've paid resulting in the Authorization to Test (ATT) is only valid for 90 days. After that date, you have to reapply (and pay fee). All of the certification requirements (successful completion of cognitive and psychomotor exams) must be completed within 2 years of course completion.
Statistically speaking, your best chances of success on the exams come the sooner you take the courses after your exams.
https://www.nremt.org/nremt/downloads/RefundPolicyATTReceived.pdf
As luke said, NREMT.org is an excellent resource, both the web content and phone representatives. It would benefit you to review the information available (including documents from your EMT course instructor and emails from NR), as I suspect many of these questions could be answered there.
Let us know if you have more questions