answer

https://www.canada.ca/en/revenue-agency/services/forms-publications/publications/t4144/income-tax-guide-electing-under-section-216-2016.html

When is your section 216 return due?

Generally, you have to send us your section 216 return within two years from the end of the year in which the rental income was paid or credited to you. For exceptions, see When is your 2017 section 216 return due?.