Hi! I've had the pleasure to deal with mononucleosis in adulthood as well. Liver and spleen suffer (the spleen will grow quite a bit) and it will lead to weeks of fever and feeling generally miserable. For me the first acute infection lasted 5 weeks. And the most lovely bit is that you can fall back into sickness later on because that sucker virus stays inactive in your body for the rest of your life - ready to hit you again when you least expect it (e.g. after exhausting activity like sports). The second time it hit me this lasted 3 weeks. yay.

I wish you all the best and a speedy recovery.

Christian