Luteal Phase?

Question ladies,
Every month for the past 5 months I have been keeping track of my luteal phase. I noticed on month 1 & 2 it was 11 days so I started taking vitamins to help lengthen it. Month 3 was 11 day and month 4 was 15 days. Now I'm not 100% sure if the vitamins really work or not so which one should I use the 11 or the 15???