Why is my lh surge last 72 hours not the normal 36 hrs? i ovulate every month

It doesn't: I'm assuming you're using ovulation prediction kits. Once your surge happens it takes a period of time for the LH to go back down below a level where it goes below the test levels indicated to be positive. I addition if you are checking the LH very often you will pick up a positive a bit before the actual surge. I tell my patients once you have a positive test to stop & save for the next cycle