There is no Canada prime minister term limit. They are allowed to serve until they either resign, die or are released by the governor-general.
The longest-serving out of all of the Canadian prime ministers was the Right Honourable William Lyon Mackenzie King. He, over several terms, served a total of 21 years, five months and one day in office. Justin Trudeau's father, Pierre Trudeau, served for over 15 years as Prime minister from April 20, 1968, to June 4, 1979, and from March 3, 1980, to June 30, 1984.
There has never been a Canadian prime minister term limit, nor does there seem to be any plan to be one in future.