Once you've been exposed to the influenza virus it can take up to 48 hours before you start to get sick from the flu virus.
Sometimes it can take up to 3 to 4 days depending on the person and the strain of the flu virus but normally it takes only 48 hours to become sick with the flu.
That's why if you've been prescribed Tamiflu you should take the Tamiflu 48 hours before the flu sets in and you should continue to take Tamiflu until your doctor says you can quit taking the Tamiflu.