Canada has introduced a new entry requirement, known as the “Electronic Travel Authorization”(eTA), for visa-exempt foreign nationals traveling to Canada by air. Exceptions include U.S. citizens and travelers with a valid visa.
1) Apply for an eTA as early as possible.
2) Wait to make any non-refundable purchases (flights, accommodations, etc) until *after* your eTA has been approved.
3) The eTA is NOT a Visa. It is only authorization to board a plane to Canada.
4) Without a valid eTA, you WILL be denied boarding by the airline.
To answer your question, I would leave 1 - 1.5 hours between flight arrival and bus departure.
If you take the Banff Airporter shuttle, you give them your flight number when you reserve.