We ran our first proper audit of short-term business travel last year, using expense and travel booking data for about 5,000 employees at an engineering software company. The working assumption in the business was that anything under 183 days in a country is safe.
It turned up 212 people with more than 30 days in a single foreign country. Among them, 17 were effectively working for a local entity that was bearing their cost, which our advisers said can remove treaty protection from day one. Several countries on the list also have payroll reporting or registration duties that start well before 183 days, and immigration status was wrong for a handful.
We now require pre-trip assessment for any country where someone will exceed 20 days in a year.