You should have one person who is assigned to your medical case anyway. So you could definitely ask him/her some of these questions. When were you nominated? Have you received any follow-up questionnaires from Peace Corps? I had follow-up questionnaires about my asthma, medications I was taking and other aspects of my medical history. It took me a few weeks to fill them out. Once I turned them back in, I think it took a few more months for the invitation to come. Luckily, they accepted my follow-ups with no further questions. But they can always come back with more questions, especially if you have an extensive medical history. Finding out who your medical contact is would help you a lot.
It is hard for me to give you advice about the timing without knowing how long you have been waiting in the medical process and if there have been any follow-ups. But placements can fall through. Is there any way you can take a sabbatical for 2 years? Or would you be quitting to go into Peace Corps? You are in a particular situation. Have you spoken with your employer, asking them any of these questions? Or is it a situation in which you don't want to alert them yet?
Unfortunately nothing is guaranteed in terms of timing. You may also be able to talk with someone at the placement office. Just keep calling and see who you can get to speak with!
RPCV, Botswana 2011-2013