The fatigue, weakness and coughs from the flu should start to go away within 3 days however the flu itself can last from 1 or 2 weeks until the flu virus is able to fully get out of your system.
There's not much you can do other than stay hydrated, get plenty of rest and take some cold and flu medicine but you should fully recover from the flu within a week and sometimes it can take as long as 2 weeks.