This depends upon what is being treated.  But for any particular problem the patient should start to improve within 2 weeks or 6 treatments.  If you’re in doubt, get a second opinion from an independent provider or specialist.
Chiropractic treatments vary widely.  Some encourage treatment programs for 12 months or longer, and may tell you that, “changing your spine takes time”.  I disagree with this approach to treatment.  Whoever treats you, I think you should critically evaluate your improvement.  Improving is not a mystery.  You will know if you are improving or not.  Your pain should reduce and your function should increase.