Apparently, over time, Mono becomes less and less contagious, but it can be spread after a period of only two weeks.
"Doctors and researchers aren't exactly sure how long someone with mono stays contagious after symptoms are gone. But it's generally believed that a person can spread the infection for many months after the symptoms are completely gone — some studies show as long as 18 months."
http://kidshealth.org/teen/infection...mono_long.html