Once I pay for a certification exam, how long do I have to take it?

I would like to purchase the CoreXM Expert + Qualtrics Platform Essentials Certification before the end of this year (for budget reasons), but I am not ready to take the exams yet. Can I purchase it and then take the exams once I am ready? Or do they have to be taken shortly after I purchase them? How long do I have between purchasing the exams and having to take them? Any help would be appreciated.