How long does LH surge last?

I've never had much luck with OPKs until I used them in conjunction with fertility drugs. In past cycles including the one when I got pregnant (ended in miscarriage) I had positive OPKs for 2-3 days. This cycle I used gonal f in conjunction with a trigger shot for my third iui (which I had today - fingers crossed!) Anyway, yesterday morning's opk was getting closer but not quite there and so was the one I took this morning. I took another opk this evening and it was completely negative. Could I have surged in the middle of the night and that's why it's not reflected on an opk test? I was cramping while in bed so I'm assuming so, it's just weird that it happened so quickly. Has this happened to anyone else? Getting blood drawn on Friday to check if I ovulated.