grants.nih.gov/grants/guide/pa-files/PA-14-042.html#_Section_IV._Application
K99 applicants must have no more than 4 years of postdoctoral research
experience at the time of the initial or the subsequent resubmission or
revision application, and must be in mentored, postdoctoral training
positions to be eligible to apply to the K99/R00 program.
按照这条,应该是指不超过4年的postdoc吧