Hi Christine!
It really depends on a number of factors and it's not like that it drops constantly with respect to time. Although it does drop eventually with time since cells are continuously reaching the end of their life span etc, the age of the child and amount of healthy cells also vary this phenomenon, furthermore we also have to consider things like infection too which can shorten the life span of the healthy cells.
So, it's really hard to tell what is the rate of haemoglobin "Burning up" at different ages.
Take care, Peace!