So How High Can I Go?
Some runners get really ambitious and assume they're getting a great workout if they do runs at a ridiculously high incline. I've seen runners and walkers at the gym set the incline so high that they're holding onto the handrails for dear life. (Those handrails are to help you get on and off the treadmill safely, not to assist you in mountain climbing, by the way.)
Just because the treadmill goes up to a certain incline doesn't mean that you should set it that high. You certainly wouldn't set the speed to the max, would you? When choosing an incline, avoid going above a 7% incline. Anything higher than that places too much strain on your back, hips and especially your lower legs and could lead to injury.
How Long Should I Run at an Incline?
I've also seen the people at the gym who run their entire distance at an incline. Continuous hill running like that is never a good idea and could lead to injuries. Think about it: Would you ever find a 3-mile hill at a 6% incline? Or, if you did, would it be smart to run up it?
You should avoid running at a steep incline for more than five minutes. You'll get a much better, safer workout if you switch between running a couple of minutes with an incline and running a couple of minutes without an incline, like in this treadmill hill workout. You could even do shorter hill intervals, like 30 or 60 seconds, to mix things up. It will make your runs a lot more interesting and prevent you from getting bored.
Also see: Video: How to Run on a Treadmill