When to spay depends on the season as well as the age of the ferret.
 
If she was born late in the year, then she will probably come into season in
the Spring.  if she was born in the Early spring, she could go into season
before the winter sets in.  it all depends on light and temperature.
 
Best bet is at four months if you want to avoid the first heat cycle, but
if you want her to be fully grown, then 6-8 months as long as the vulva
does not swell.
 
Unfortunately, there is no perfect answer for your question.
 
Pam Grant / STAR*
 
ps - all ferrets have scent glands unless removed.  It is just that males
smell more when in season than females do.
[Posted in FML issue 1357]