How long is spin training good for in the USA?
CFI candidates receive a spin training endorsement with no expiration date. According to AC 61-65E (the endorsement appendix), it should look like this:


  46. Spin training: section 61.183(i)(1).
  
  I certify that (First name, MI, Last name) has received the required training of section 61.183(i). I have determined that he/she is competent in instructional skills for training stall awareness, spin entry, spins, and spin recovery procedures.
  
  
  /s/ [date] J. J. Jones 987654321CFI Exp. 12-31-05
  


The basis for this is that 61.183 doesn't specify a time period in which the endorsement must be received:


  §61.183   Eligibility requirements.
  
  To be eligible for a flight instructor certificate or rating a person must:
  
  
    [...]
    
    (i) Accomplish the following for a flight instructor certificate with an airplane or a glider rating:
    
    
      (1) Receive a logbook endorsement from an authorized instructor indicating that the applicant is competent and possesses instructional proficiency in stall awareness, spin entry, spins, and spin recovery procedures after providing the applicant with flight training in those training areas in an airplane or glider, as appropriate, that is certificated for spins; and