Friend..answer to your question
Medical test validity applies only to your sending the I-485 application. ie., You should file your I-485 within one year from your Medical test date. After that even if INS takes 100 years ( just kidding ) to adjudicate a case, your medical test is valid.
Again reiterating my answer, if your medical test date is within a year of your I-485 application date, then your medical test has indefinite validity.
Hope this helps. Good luck.
Srirenga @ Tamizhan