If you have mono and have a fever along with the mono the fever usually lasts 10 days.
Then it takes around 4 to 6 weeks to get over mono once you've gotten it.
But even after the 6 weeks when the mono goes away you may still feel tired for another 6 months which is normal and if it still hasn't gone away within 6 months you should go to urgent care or see your doctor or if symptoms get worse.