Once you get the fever during mono the effects of the mono will last from 2 to 4 weeks.
The major symptoms of mono typically only last around 2 weeks and then they start gradually going away and you should begin feeling better within 4 weeks.
But although the mono symptoms should go away within 4 weeks some people can normally have fatigue for months and even years after the mono has gone away.
But if the mono symptoms other than fatigue don't go away within 4 weeks or the mono seems to get worse you should see a doctor just to be safe.
But most times you should be feeling much better other than being fatigued within 4 weeks but sometimes within 2 weeks.