You can stop taking Tamiflu for your flu at anytime you'd like but it's best to continue to take the Tamiflu for as long as the doctor tells you too.
If you stop taking the Tamiflu too soon you risk having the flu come back soon after.
So to allow the Tamiflu to work to it's full potential it's advised that you take the Tamiflu until the time your doctor tells you that you can stop taking it.