When you have the flu virus you should take off of work and stay home and rest for at least 4 to 5 days until you're feeling better.
When you have the flu you will usually be feeling pretty bad and fatigued for at least 3 to 4 days after the flu virus infects you.
So it's best to stay home from work for 4 to 5 days after you get the flu as you won't be feeling too good anyway and won't be very productive at work when you have the flu.
When you have the flu you're most contagious for 3 days after the flu symptoms come on and after 3 to 4 days of the flu you usually are no longer contagious and can return to work.
I stay home from work for at least 5 days after I get the flu to make sure I'm feeling better.