This is different for everyone as each individual will experience waist training differently. We strongly recommend purchasing two waist trainers at one time as you will most likely go down in size within one month.
We recommend moving down in size when the waist trainer is no longer snug. If it feels loose and your measurements with the waist trainer on are the same or bigger than your waist measurements with the waist trainer off, then it is likely too big.
We recommend watching this video for deeper insight on this topic