There is NO firm time table for the life of a balloon.  All latex balloons will deflate.  Latex is a porous material and as helium is lighter than air, the helium will disapate through the latex.  A standard size helium filled untreated balloon (11" - 12") will float average 18 - 24 hours.   A treated 11" - 12"  balloon will float average approximately 2 weeks.  Float time is also dependent upon the temperature.

Mylar (foil) balloons will float an approximate 3 - 4 weeks.  Plastic Bubble Balloons will float 3 - 4 weeks.