As long as you're feeling strong enough and have the energy to exercise after the flu you can resume exercise right after the flu.
I exercise as soon as possible after the flu and if I'm feeling energized enough I will also take a walk if possible before the flu has gone away.
Exercise is good for the flu and helps boost your bodies immune system.
I also find that getting outside into the fresh air helps me get rid of the flu much faster as well.
Being stuffed up in the house keeps the flu virus lingering around but being outside when you have the flu you get to breathe in some fresher air which helps you feel better and recover from the flu much sooner.