If I have an OPK that's positive the day before, it's still positive with FMU. My surge seems to last two days, but everyone is different.
I usually test 2-3 times a day when I get a positive or "almost" positive. So FMU, sometimes afternoon if I'm home and evening.
Also, I started using a digital to validate the Wonfos. You use less digitals that way, but takes the guess out. And who doesn't like that little smiley face??