Since Mono is often called the kissing disease- I'd bet itsc ongagious thru sex.
The answer to your question --- they don't seem to know...
How long is mono contagious
Mono, short for mononucleosis, is caused by an infection with the Epstein-Barr virus (EBV), a virus in the herpes family of viruses. As with other herpes viruses, EBV stays in your body for the rest of your life, although it won't cause you to get mono again. However, there are other viruses you can catch that can cause you to have a mono-like illness, too.
Frequently referred to as the "kissing disease" because you can catch it from the saliva of someone who has the virus, mono can also be contracted by sharing a toothbrush, drinking straw, or eating utensil with someone who has the virus. Don't worry about giving your friends or family a quick peck - it takes more than that to catch it.
Once you get the virus, you generally don't have any symptoms for 4 to 7 weeks. When symptoms begin, you'll probably have a fever, sore muscles, headache, sore throat, and loss of appetite. Two indicators that you may have something more than the flu are being tired all the time and having swollen lymph nodes (glands) in your neck, under your arms, and around your groin. Some people also have an enlarged liver or spleen. A blood test, known as the Monospot test, can help your doctor determine whether you have mono.
Mono can't be cured and needs to run its course naturally over about 2 to 4 weeks - and some people feel tired for several weeks longer. There are some things you can do to feel better and help yourself recover. First and foremost, get plenty of rest. Drink lots of liquids, such as water, juice, soup broth, and tea, and try to eat a well-balanced diet. You can take nonaspirin pain relievers, such as Tylenol or Advil, which will help your headaches and sore muscles while reducing your fever. (Do not take products containing aspirin, as they may cause Reye syndrome, a serious and sometimes fatal disease.)
Answering the big question about how long you are contagious is tricky because doctors and researchers aren't exactly sure. You can spread the infection for the entire time you have symptoms and probably for a few months after your symptoms are completely gone. It may be a good idea not to share that ice cream soda for about 6 months - until after you start feeling better. You also should avoid open-mouth kissing, too. Remember, that might be how you caught mono in the first place!