It is the length of time between when you ovulate to when you start your bleed. Have you used OPK's at all? they can help you predict when you are ovulating if you don't have many signs like cramping or EWCM. If your cycle has always been 31 days, then that is your 'normal', just as mine is 34-38 days and some women have 28 days. But all women should still have a 14 day LP, but those of us will have longer between the start of our bleed (cycle day 1) and when we ovulate.