The seven years represents a period long enough to cover the business cycle. At one time the legislation referred to setting the rate so that the EI account would balance over the business cycle. By using seven years, this legislation sets a fixed timeframe. It's a rolling seven years so it doesn't really matter when the business cycle begins or ends, but it's long enough to cover a period of decline and the rebound in the economy.
On November 18th, 2013. See this statement in context.