You can unlock a maximum of 3 Medals per mine. Your best completion time determines how many Medals you unlock.

Completing the mine (any time): 1 Medal
Completing in under 72 hours: 2 Medals
Completing in under 24 hours: 3 Medals

Replaying a mine will not give more Medals unless you complete it faster than before. For example, if you complete the mine in 5 days, you will get 1 Medal. If you replay it and complete it in under 24 hours, you will get another 2 Medals, making your total Medals for that mine the maximum of 3.