After being exposed to the flu virus it takes around 24 to 48 hours to get sick with the flu.
If you're prescribed Tamiflu you should take the Tamiflu 48 hours before the onset of the flu for it to be effective.
The Flu will then typically last for 7 to 10 days after day 10 you should no longer be contagious with the flu and can then return to school or work.