The result of the medical exam is valid for 1 year. This ONE year is counted from the date you took the exam. If they visa office think they cannot issue the visa by the date that is 1 year from that date, or if they think you cannot enter Canada by that time, they will ask you to take the second medical exam.

For example, if you took your last ME on May 31, 2000 and you are applying for a student visa starting Fall, 2001, you have to take the ME again.