How long after you have taken Tamiflu is it still effective?

When my 9 year old caught the flu the first week in March, our entire family took Tamiflu. My son kicked the flu in record time and the rest of us avoided it completely. It is now almost two months after the fact: How long do we we have immunity from the Tamiflu? My husband travels a lot and I am hoping with the swine flu that is all over the news, he will have some protection.