Prepositions are used when we describe a specific point in time or a period.
| Klokken er ti minutter over fem. | The time is ten minutes past five. |
| Om lørdagen går jeg altid en tur. | On Saturdays I always go for a walk. |
If the expression answers the question how long, we always use i.
| Jeg har boet i Danmark i 6 år. | I have lived in Denmark for 6 years. |
If the expression answers the question how fast, we always use på.
| Han kan løbe 1 km på 10 minutter. | He can run 1 km in 10 minutes. |