From the Canada AIP:

"In the morning, civil twilight begins when the centre of the sun’s disc is 6°
below the horizon and is ascending, and ends at sunrise, approximately
25 minutes later. In the evening, civil twilight begins at sunset, and ends
when the centre of the sun’s disc is 6° below the horizon and is descending,
approximately 25 minutes later. The number of minutes varies depending
on the latitude and the time of year.
"

Horizon may be affected by relative terrain elevation in the crepuscular direction.