Written tests are only good for 24 calendar months before they must be applied to a certificate (or lost). Spin training is a requirement of the flight instructor certificate. Does the required spin training have the same limitation of life as a written test? Or, is it like an endorsement where it's good "forever"?
CFI candidates receive a spin training endorsement with no expiration date. According to AC 61-65E (the endorsement appendix), it should look like this:
The basis for this is that 61.183 doesn't specify a time period in which the endorsement must be received: