that number is determined by the case, mainboard and psu location on the case
could be short as 5 inches or long as 20 inches
tell us what you bought to know what you have there, usually you need at least 15 inches cables on a mid tower to have space to handle the cables, you have to do measurements anyway, a preassemble will tell you more, do measurements and overprovision, thinking on possilbe problems in length, prefer to do them longer, shorter cables for a psu are a no go, longer can be handled in one way or another