When is the right time to get a Beta test?

Hi, and thank you for your time. My surrogate was due for AF today and it was a no show so far. She is always like clockwork with a 24 day cycle. We may have miscalculated the ovulation day because we assumed that the OPK was defective, and she has had a BFN today and every day before today. When would be a good time to get blood work done? Did inseminations on July 14th, 16th, and 17th.