The flu can be deadly in some rare cases but most times the flu is just annoying and makes you feel really bad.
Most cases of the flu are not really severe enough to warrant a hospital or doctors visit and should go away within 1 week to 2 weeks.
If the flu lasts longer than 2 weeks or you start getting a much higher fever or the fever doesn't go away or your flu symptoms start to become worse then you should be concerned and go to urgent care of the emergency room just to be safe.
Muscle Aches, Body Aches, Fever, Coughing, Headaches, burning eyes etc are all normal with the flu and should go away within a week or two.
A flu that lasts longer than 14 days should be checked out by a doctor because you might have a more severe strain of the flu.