I have seen people on here say that after six months they had to do a new medical. I've also seen people say that they had to be in the United States before the six month period is up...that's from the date of the medical, not the date of issue for your visa. I made sure my mother-in-law entered the U.S. within six months of the date of her physical and had no issues. (Her interview and visa issuance went quickly.)
An immigration attorney mentioned this last week on social media, and I went to look up the USCIS statement:
https://www.uscis.gov/newsroom/alerts/uscis-announces-new-guidance-on-form-i-693-validity-period
U.S. Citizenship and Immigration Services today announced (PDF, 317.28 KB) that any Form I-693, Report of Immigration Medical Examination and Vaccination Record, that was properly completed and signed by a civil surgeon on or after Nov. 1, 2023, does not expire and can be used indefinitely as evidence to show that the applicant is not inadmissible on health-related grounds.
This policy was published at the start of April 2024, so it may be that not every IO is up to date on it. That same article mentions that previous policy used a two-year or a three-year expiration date for I-693s.
My own experience is that if you're in the U.S. before your medical is more than six months old you're fine.
Regards,
Vicky's Mom