I would say how a person can be considered a serial killer for example. On the first day, the person commits his first murder then one week later he commits another murder then another week later he commits another murder then he continues doing the same in another week.

But then that would make for 3 victims in the span of 21 days, which would be a spree killer by my definition, which is <30 days

Actually a person who kills 3 or more people for 2 weeks or more than 2 weeks can be considered a serial killer