Most age gaps between siblings in Denmark are 2-5 years. Most of my friends from school had siblings with those age gaps. I personally consider it a large age gap if it's 8 years or more, but that could just be me.
Edit: I am referring to the age gap between the siblings 'next to each other' in the order of which they were born. Not the age gap between the oldest and youngest sibling in a family of five kids