Does your LH surge look the same each cycle or does it vary?
Last cycle was my first cycle using OPKs. I tested and got an almost positive OPK followed by a positive OPK and the next morning the test line was much lighter. So it was a very short LH surge.
This cycle I have had 5 almost positive OPKs over the last 3 days. 1 of them I think was as dark as the control line.
Mine is usually the same each cycle. It's always around the same cycle day and it lasts for 24 hours tops. I only test with digital OPKs because I find it's easier and leaves less to interpretation. I tried both kinds and like the digitals better (they are more expensive though).
But I guess each person is different so your LH could vary with each cycle.