It's different in different provinces. In Ontario, it's from Grade 9 to Grade 12, typically four years but some students choose to take an extra semester or year to fit in more courses of interest.
In some other provinces (Alberta for example), high school is grades 10-12, and it comes after junior high of grades 7-9.
In Quebec, secondary school is grades 7 through 11, and it is followed by CEGEP, which is kind of similar to a sixth-form college in the UK - CEGEP can either be a step before university or a place to finish one's education without going to university.