Rebooking Disrupted/Schedule Changed Flight

Last year, i booked a flight for march31. Last week, may schedule change daw and pwede i-rebook or convert to travel fund. Given tuloy talaga lakad ko, I'm gonna rebook it. However, may hinihintay pa ako sa client ko on whether they need me on april 14 or 15. Question is: until when can i decide/rebook this flight? 7 days before? 3 days before? 2 days before? 1 day before? 2 hrs before? Thanks!