I guess you're looking for the half-life formula
If the half-life is H days and you have an amount N at the start then after D days you'll have
N
--------
2^(D/H)
Where the ^ symbol is for raising to the power. So, as an example, if you inject 500mg of something with a half-life of 8 days then after 4 days you'll have
500
---------
2^(4/8)
which is 353.55