Mono causes months of extreme fatigue when symptomatic (first study I found says 72% of infected patients had fatigue), this study says 4% of high schoolers met the conditions for Chronic Fatigue Syndrome 24 months "after infectious mononucleosis" (unclear if that's after diagnosis or what). In that study 11/13 cases improved between 12 and 24 months, but 2 were worse. 24 months out was the longest I could find: lots of studies showed extreme fatigue at 6 or 12 months but that seems more like a slow recovery that chronic infection.

Mono is unlikely to be a good model for COVID-19 because the infection is permanent, like chickenpox, whereas I believe coronaviruses are typically fully cleared.

Pandermix, an H1N1 vaccine, is general considered to have led to an increase in rates of narcolepsy, especially in humans. None of the studies I found even mentioned relative risk except for this critical survey, done by people receiving money from the vaccine manufacturer.