There are 30 Cans which have milk.
1. Among those one can got fully poisoned.
2. So even if a man taste a 1/8th of single drop of that can's milk he will die exactly after 12 hours.
How many minimum number of rats you need to test and find the poisoned can?

answer is 4. 
If a man taste the 8th part of one drop he will die exactly after 12 hours. For the same thing rat will die less than 12 hours. because of it's low immunity power. 
Step1: Take first 15 cans and do as shown in figure.

After 12 hours:

1. If first Rat only dies then can-1 is poisoned. 
2. If second Rat only dies then can-2 is poisoned. 
3. If 3rd Rat only dies then can-3 is poisoned. 
4. If 4th Rat only dies then can-4 is poisoned. 
5. If 1st Rat and 2nd Rat will die then can-5 is poisoned and so on. 
at last if no rats are die then no can is poisoned. In this cases, proceed step 2. 
Step2: Do the same thing for the next 15 cans with  those rats and find the poisoned one.