It is difficult to say how long the labia majora and minora should be, given that everyone has different labia. The labia can be asymmetrical or symmetrical, or they may be longer or shorter, with one side hanging out lower than the other. The labia minora (the inner lips) can also be bigger or smaller than the labia majora. There should be no length that is one of concern, unless they are causing you physical discomfort in any way. Most OB/GYNs would say they have never seen a labia that isn't normal! The only way to shorten the labia is with surgery, which should be done by a licensed professional who has been trained to do the procedure.
You can find out more about the diversity of labia at The Vulva Gallery.