New to the OPKs and just used on for the first time this cycle.

Went to have an abdominal ultrasound done today and the tech told me she could see my follicle and it measured 2.5 cm (mature) and I could O anytime.

Took a OPK test this afternoon, LH surge, took one now at midnight and its negative, so I can only assume the surge is done.

My question is sex. Some people are saying that the sperm may not get to the egg fast enough (within 12 hours), or that I should have had sex prior to my positive opk. My question is when should I have sex again, we did tonight so that's good I guess. Should we continue for a few days?