How long would it take to ride a mile at 5 mph on a stationary bike?

Comparing stationary cycling to real-world cycling is pointless in terms of distances. IRL you might have headwind, tailwind, a hill to climb or descend from and lots of other things that'd affect the distance covered. 5 minute is 5 minutes, that's about it. Besides, the speed of a bike ride can be anything between 5-35 MPH. Assuming an easily reachable 12 MPH 5 minutes of riding would see you covering one mile, but the result could well be entirely different. And while 5 minutes is indeed better than nothing it's way too short to have any significant health/weight benefits by itself.