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According to Government of Canada, duration (for a business visitor*) is at the discretion of the border services officer:
A border services officer at the port of entry in Canada will determine how long you can stay in Canada. Most visitors are allowed a six-month stay from the day they entered Canada.
*A business visitor is someone who comes to Canada to take part in international business activities without being part of the Canadian labour market. Business visitors usually stay in Canada for a few days or a few weeks but are able to stay for up to six months.
In case of doubt, that means "six months straight". Each visit is assessed separately.