the luteal ph is usually pretty regular, so once you have found out what yours is, it shouldnt change by more than a couple of days, if your cycle is irregular its usually the first part (follicular) that changes, so on month you may ovulate on dat 11 and the next month day 16, but you luteal ph should be the same, most doctors like to see a luteal ph of at leat 12 days although some say 10 is ok, if you find yours is shorter than this then its known as a luteal ph defect, vitamin b6 can help lengthen this part of your cycle and so can vitex :)
The luteal phase is the second part of your cycle...from ovulation till your next period starts. The first part, from day one of your period until you ovulate, is called the follicular phase. Hope that helps.
:-)