Curiosity killed the cat?
Python 3, 33 bytes

lambda a,b:a-a*.5**(6048e5/b)&gt;9e3


Explanation:

         6048e5         # number of milliseconds in 1 week
               /b       # half-lifes per week
  a*.5**(        )      # mgs of substance remaining after 1 week
a-                      # mgs of substance decayed after one week
                  &gt;9e3  # return true if more than 9000mgs has decayed in 1 week


Try here