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A visitor's visa is generally stamped for 6 months. What's to stop a visitor
from crossing into, say Canada, just prior to the end of the 6 months and
re-entering for another 6 months? How long can this be done?

Any replies welcome :-)

crg

It sounds good in theory but remember that a visa only allows you to ask permission to enter the US, and does not guarrantee your admission. If you can't prove to the border agent that you have a residence/job/life outside the US and ties to a foreign country that you do not plan to abandon, then you may find yourself stuck in Canada.

Also, it's not unheard of for the Canadian border officer to become concerned that you won't be allowed back into the US. They have been known to deny admission in these cases. You may also need a Canadian visitors visa before seeking entry.

So, to answer your question about what is going to stop you, it's the officer at the border with the visa cancelation stamp and the authority to order you removed.