How severe is infectious mononucleosis?

Usually not: Most people with mono are much better after 1-2 weeks but may have times of fatigue on and off.

Depends: The core symptoms of fever and sore throat often resolve within a couple of weeks; however, the fatigue can last for months. Unfortunately, there is no efficacious way to treat the fatigue, as it's a sequela of the infection and not evidence of continued infection; that is, even if you had an antiviral for it, there would be no benefit.