Always apply for the eTA as soon as possible. Normally the authorization is approved within very short time, but in case that your authorization was denied you can expect additionally waiting time for your travel authorization.
eTA Canada – Valid for up to five years or until the passport expires, whichever comes first. If you get a new passport, you need to get a new eTA.
eTA Travel – With a valid eTA, you can travel to Canada as often as you want for short stays (normally for up to six months at a time).