I'm just finishing up AF right now.
These were my last 6 month cycles:

Dec 26, 2014 - Dec. 30, 2014 (Currently on day 5)
Nov 11, 2014 - Dec. 5, 2014 (26 day cycle)
Oct 28, 2014 - Nov 2, 2014 (33 day cycle)
Sept 24, 2014 - Sept 29, 2014 (34 day cycle)
Aug 14, 2014 - Aug 19, 2014 (41 day cycle)
July 14, 2014 - July 19, 2014 (31 day cycle)

Average is 31 days, but I'm alarmed at my shortening cycles?!
AF generally stays 5 - 6 days.
I only have 10 OPK tests to use. Using the Clear Blue Advanced Digital.

I looked at my OPK chart and it says that:
26 day cycles - start testing on Day 7
31 day cycles - start testing Day 11

What should I do? When should I start?

ETA: Average Luteal phase about 12 - 14 days.