I will say it in a simple way:
Say you go for your medicals on october 1st 2009 and the doctor sends the result off to the HIC and it was received by the CHC on the 10th of october 2009. Your visa will be valid until the 9th of october 2011.
The valid from date is different depending on visa offices. Some have been given 10months, 6months e.tc. I know of a guy that was given 4days ONLY. He did his medicals on july 24 2008. The doctor sent it off to CHC london on august 4th 2008, it was received by london CHC on august 5th 2008. His visa was issued on the 27th of july 2009 and was valid till AUGUST 4th, 2009. I hope this answers your question. If you don't land and your visa expires, you have to re-apply if you still want to immigrate. No guarantee for visa being issued and your application will be processed based on the new rules in place.
Good luck.