Your best cardio workout is going to come from faster spinning, which means you're in lower gears on the bike. Higher gears require more power to push, so that would require burning more calories.
However, there's more to it than just getting in one gear and spinning for cardio. When you're outside you will run into things like headwinds, tailwinds, sidewinds, uphills, downhills, etc. You need to be able to know which gear to shift into in order to best address your current situation.
For example the other day we were riding bridges (no hills here in Florida) and there was a steady wind coming out of the west. I dropped my gears a couple of times for easier pedaling on the way up, but just before I hit the crest, the wind hit me full on. I had to get into higher (harder to push) gears immediately, especially for the downhill. The fact that I was riding fast downhill plus the headwind that kept trying to push me around required more control over the bike. The higher gears gave me that control. That and sitting back on my seat while feathering the brakes helped, too!
So the basic answer to your question is that you get more of a cardio workout in lower gears, but you need all of your gears in real world on the road ride. With regard to your question as to how far people ride to get the best cardio workout? I think that depends on the person. A fitter person obviously will go farther to get the same carido benefits as a not so fit person.
"God gave you your body as a gift, so you should take care of it." - My Mom