I,m not sure if that means you have to be coming to live permanently, or if there is a time limit...say 5 years???
Well, I would have expected that when you had the thought about how long a person would stay in Poland and how the taxman would be able to determine their length of stay, that your common sense would kick in. Apparently not.
So, no, you are not tax exempt based on how long you stay and your car most certainly won't be treated as a household item or clothing article that you bring with you when moving to Poland.
The amount you have to pay depends on how old the car is. If it is older than 6 months you will only pay excise duty which will depend on several factors such as age of car, engine size etc.
If the first registration of the car was less than 6 month ago you pay normal VAT rate of the purchasing price on top of that.
In any case whether or not the taxman wants your money for your imported car does not depend on how long you will stay in Poland.