when travelling to the us how much of my purchases are duty free?

Question

Travelling from Saskatchewan to North Dakota

Answer 1

I will assume from the way your question is worded that you live in Sask, and are visiting in ND. Based on that assumption, this website will give you all the information you need:
http://cbsa-asfc.gc.ca/E/pub/cp/rc4044/rc4044-e.html#P026

In short, if you are out of Canada 24-48 hours; $50 (see note on above website tho)
48 hours to less than 7 days: $200
a full 7 days or more: $750

These amounts are per person.

See the website linked above for full information on exemptions, and this links below for more information on traveling details.
http://cbsa-asfc.gc.ca/E/pub/cp/rc4044/rc4044-e.html

Answer 2

That depends if you are a US citizen or not. Just as for crossing into Canada, the limits are different for citizens (or legal resident aliens) versus not.