Validity of the eTA Canada
For the validity of the eTA Canada, three things are of importance. The eTA has a total validity period, which starts from the moment your application is approved. Furthermore, there is a staying limit per visit, which starts the moment you arrive in Canada. Finally, there are a number of general requirements which you have to meet to make sure this travel authorisation remains valid for the entirety of your stay in Canada.
Total validity duration of the eTA
The eTA has a total validity duration of five years. This period of five years starts from the moment the eTA is granted. Even if you indicate that you are not due to fly to Canada for six more months, the validity term of five years starts right after you receive the confirmation e-mail that the eTA has been approved. Due to the long validity period (5 years) this generally does not cause any problems. What you do need to keep in mind, however, is the expiry date of the passport that was used to submit the application. An eTA is always linked to the passport number that is used during the eTA application. As soon as the validity of the passport expires, the validity of the linked eTA expires as well.
New passport
As soon as you apply for a new passport, you also need to submit a new Canada eTA application to still be allowed to fly to Canada. The reason for this is simple: when boarding your flight to Canada, the airline will run your passport through a scanner. This will cross-reference your passport with a list of all approved and valid eTAs. If you are carrying a new passport, then this passport number will not bring up an eTA, and you will not be allowed to board the plane. It is not possible to transfer an existing eTA to a new passport, you are required to apply for a completely new eTA and fill in your new passport number.
Validity duration per trip to Canada
Aside from the total validity duration of five years, there is also a maximum length of stay in Canada. If you are travelling to Canada on an eTA, you can stay in the country for a maximum of six months. If you stay any longer, you might experience problems when leaving the country. Although you will most likely be allowed to leave, you might not be allowed to return, even if the validity duration of five years has not expired yet. Furthermore, not adhering to the staying limit of six months per visit can result in any future eTA or visa applications for Canada being denied. If you follow the rules as normal, you can use an eTA to travel to Canada multiple times during its five years validity term.
Aside from the total validity duration of five years, there is also a maximum duration of your stay in Canada. If you are travelling to Canada on an eTA, you can stay in the country for a maximum of 6 months.
General requirements for validity
If you applied for an eTA and received a confirmation that the application was approved, this does not mean that the eTA is unequivocally valid and remains so. A number of things can lead to an approved eTA being declared invalid, and you being denied entry into Canada (or the plane to Canada). This concerns the following things: wrongly filling in information in the application form, an invalid passport, not meeting the requirements of the eTA, and changing the personal situation of the traveller.
Wrongly filled in information
If incorrect information is filled in during the application, this can lead to the application being rejected or it being unjustly approved. If it is rejected, you can submit a new eTA application. When it has been unjustly approved, a larger problem can occur. If an eTA is approved based on faulty information, this can make a traveller assume that they are allowed to go to Canada, even though the mistake can be found at any time, stopping the trip dead in its tracks.
Depending on the nature of the error, this can lead to different outcomes. Canadian customs always has the final say in these matters, and the final decision is based on the assessment from a customs employee. This decision can be influenced by various things; for instance, if the employee suspects that you intentionally filled in wrong information, or whether you accidentally misspelled a small detail (like the address of your employer). If the error is found before departure, for example at the airport, then the airline can stop your trip. If you find that you have filled in wrong information during the application procedure, it is extremely important that you submit a new application, in order to still travel to Canada.

Invalid or expired passport
If you have a passport that is no longer valid on the day of departure, your airline will not allow you to board the plane. If your passport is no longer valid during your stay in Canada, this can lead to you being denied entry in the future. If your passport expires within the validity term of five years, and you are not in Canada at the time, you can submit a new application with your new passport before being allowed to fly to Canada again.
Not meeting the requirements
Aside from the validity duration and requirements for the passport, it is important that you meet the general requirements to travel with an eTA. The most important requirement is that you are travelling for business, tourism, or to visit family or friends. Furthermore, you do not meet the requirements if you are addicted to narcotics, have any serious transmittable diseases, have a criminal past, have serious psychological problems, or in any other way could pose a risk to the safety and health of Canada and its inhabitants. Read more about the requirements.
Changing the personal status of the traveller
It might happen that your personal situation changes between the moment you fill in the eTA form and the start of your trip to Canada. Despite having filled in the form truthfully and in good conscience, the eTA might still lose its validity. For instance, if you filled in you never committed a crime but are then put on trial for one after applying for the eTA, the eTA will lose its validity. Even if you are permitted on board of the plane despite your change of status (for example, because neither you nor the airline realize the eTA lost its validity), this can still cause problems in Canada or for future visits to Canada.