On your arrival to Canada, the border officer may or may not put a stamp in your passport. If your passport stamp has no additional notes or your passport did not get stamped upon entry, pursuant to section 183(2) of the Immigration and Refugee Protection Regulations, you are allowed to stay in Canada for a period of 6 months from the day you entered Canada or until your passport expires, whichever comes first.
However, at his/her discretion the border officer may decide that you are allowed to stay in Canada for a period shorter than 6 months, in which case he/she will make a note on the stamp in your passport, recording the date by which you will be required to leave Canada.
If you want to stay in Canada for a period longer than authorized, be it 6 months or less, you have to apply to IRCC for an extension of your stay, and IRCC recommends making such an application at least 30 days before your status expires. You can continue to stay in Canada while your extension application is being processed, on what is called ‘implied’ or ‘maintained’ status. Should your application for extension of status be granted, you will receive a new document from IRCC confirming the new date by which you must leave Canada, unless you apply for another extension. Importantly, although there is no limit on the number of times you can apply for extension of status as a visitor, the decision on such applications will be up to the IRCC reviewing officer’s discretion.