How many cartons of cigarets can i buy in usa duty free and take to canada?( what if i exceed the limit?)?

After 48 hours you are allowed to bring back Tobacco 200 Cigarettes and 50 Cigars and 200g of Tobacco, it does not matter how long you stay after the 48 hours period, it is the same for 2 days or 40 days. If you exceed the limit you can bring them into Canada however they will be subject to duty which can make them quite expensive.