Herxheimer reactions were first characterises in syphilis infections a very long time ago - maybe around 1900 - and have since been found in many other spirochete infections - including lyme disease, tick borne relapsing fever, yaws, etc etc - and some other bacterial infections also.
according to the literature they can start within 2 hours of antibiotics
eg
https://www.ncbi.nlm.nih.gov/pmc/articles/pmc5239707/In a prospective study of 33 pregnant women in Texas with syphilis, who were treated with benzathine penicillin, 15 (45%) developed a JHR that was more common in primary and secondary infections than in latent infections; it started 2β8 hours after therapy, peaked at 6β12 hours, and resulted in fever and uterine contractions in most women, resulting in delivery of three infants with congenital syphilis.13in my own experience - the stronger the dose - be it herb or abx - the quicker the herx like effect comes on - so for instance - if i start having herx within a few hours i know its going to be pretty strong - whereas other milder ones take up to 3 days to show up.
i do think there is something odd and as yet unexplained going on in terms of the actual cause of herxhiemer reactions. Apart from the commonly provided explanation of bacterial die off toxins causing detoxification overload and so sick feelings/symptoms
in lyme disease this phenomenon of herx like effects can last weeks or even indefinitely - suggesting there are other mechanisms in play - it is also highly variable from person to person - one may have no herx at all - yet get better - another may have herx like reactions for years - with the same infection and on the same drug - again suggesting there are unexplained mechanisms at work. some immune inflammatory dysregulation effect seems to be likely.