My niece has the flu we think and wednesday is christmas. how long does the flu last?

Hard to guess: Nobody really knows when one person stops being contagious with influenza (with or without Tamiflu treatment). One week out from the start of the illness (one can count from the first day of fever, aches, or cough) is safe, unless a person had a more severe illness or still has a lot of cough. In a more severe case, one can wait longer, such as until 3 days after no fever nor excessive cough.