Pundit,
You have 1 year from the date of the completed Medical Examination (the date the CS signed it) for USCIS to receive it and be considered valid.
Once received by USCIS, it is valid for 1 year from the receipt date for adjudication purposes.
So, if the Medical Exam was carried out in early June 2015, it would be valid to send to USCIS until early June 2016. If USCIS received the I-693 in early June 2016, it would be valid until early June 2017 for adjudication purposes, so it is possible to stretch the validity to nearly 2 years if the cases do not become current this FY.
See 4. Validity Period of Form I-693 (Including Use of Prior Versions) in
this extract from the USCIS Policy Manual.