Whenever I have the flu the most severe symptoms of the flu such as muscle weakness, body aches, fatigue and fever subside within 3 days.
And within 1 week I'm usually fully recovered from the flu but if the flu virus is more severe then sometimes I'll feel week for up to 5 to 6 days and fully recover from the flu within 2 weeks.
Most times the flu I have goes away within a week though but no matter what the flu should not last more than 2 weeks and if you still have the flu after 2 weeks you should go see a doctor because you could have a weakened immune system or a more severe strain of the flu.