Once you register as a candidate I believe you can have up to one year for the CC, if you'd like to extend that (it frankly should be ample) then you need to pay fifty dollars.
You can book the exam anytime, though if you fail respect the timeouts.
The free exam and training process will probably make it clear as you go through it:
One Million Certified in Cybersecurity – Free (ISC)2 Certification Exams (isc2.org)
