Zero minutes, Incredibly boring. If you need cardio go outside and go for a run.
Nah.
Treadmill succeeds where outdoors fail.
You're half right. It can be boring.
Boredom is your mind or body not being stimulated and giving you the cue "just stop" "do something else" "what do you have to prove"
Once you overcome that and build that mental strength, anything becomes easy.
When you run outside there is psychological stimulation. You run past trees, dips, grass, various markers or set points that let you know you're going somewhere or you've gotten somewhere.
You build that mental toughness on the treadmill. Even marathoners will tell you that.