There is no real rule of thumb for the correct length of stirrup leathers. A common way to determine the right length in advance is to position the stirrup leathers away from the saddle and along your arm. For the length to be roughly right, the end should be at your armpit.
After mounting, however, you should check this again, according to the discipline you are riding. If the stirrups are set a little shorter for jumping, you may want to adjust them for dressage, where it's important that they are neither too short nor too long.