6 months validity from Day 1 medical to use it as a requirement for Point of Entry. So if you have your medical April 1 and 2 - and you are cleared, then your interview with USEM is April 30, or May 30, or June 30... If approved for Visa, your visa will have expiry of around November 1. which means you have to enter US before that date.
Scenario - your medical is April 1 - you are sent for sputum collection scheduled April 17,18,19.. your will be given a schedule to return to SLEC for vaccine around June 14 (it takes 8 weeks for s sputum culture).. If you are called earlier than June 14, it is possible your sputum test return anomalies. Otherwise, you get your vaccines/medical completion on your Return Day. If you see you are closing to your return day and no calls received, go book/watch your interview slot probably a week after return day (my hubby had his interview the day after Return day - we advised SLEC of interview schedule so they can send the file to USEM on return day to be ready for an early morning interview). Once you got issued a visa, USEM can only issue you visa with expiry around November 1, which is your 6 months from Day 1. You then need to plan for CFO next then fly out and enter before your visa expiry.
Hope this helps!