Smittie!! What? You don't get charged duty?? I am coming shopping with YOU!!
Here's how it is supposed to work (in theory).
If you go across and come back the same day, technically you do not get any exemptions. Having said that, often I tell them that I bought something worth $100 to $200 and they will wave me through and tell me to have a nice day.
Having said that I have also decided several times to declare honestly the approximate amount and then I go into the little office where they calculate duty and taxes and I pay up. Ugh. But better than being arrested.
If you go for 24 hours or more, you get a $200 exemption. Again, you can often tell them you bought a bit more and they'll let it go. But not always.
If you go for 48 hours or more you get to claim $800 worth without duties or taxes. Typically they don't even ask questions if I've been gone that long, or at least they don't ask seriously. I tell them I bought a few articles, they ask me to estimate the $ amount, I lowball it, and they say fine.