If one is concerned about recent exposure to hsv, how long to wait before blood test to be sure?

6 weeks: There are basically two ways to confirm herpes exposure. One is to swab an active lesion while it is present. This test comes back in a few days. The other way is to draw an antibody. It take 6 weeks after exposure before your body to form antibodies. Six weeks after exposure if your antibody test is negative then you did not acquire it.

Three ,months: Three months should be sufficient, 6 months if you wish to be more certain.