I'd disagree about the 5mph vs 30 mph. I don't use a trainer or stationary bike enough to relate to them. On a bike I can do slow speeds at a very high cadence and will have a very high cardio benefit. I can do faster speeds at a lower cadence and be below cardio benefit levels.

On my wife's stationary bike, it gives me a target HR when set up for my weight and other metrics. So if I cycle for a specific time to that target HR, then I think time will still win out. Or you can just set your own target HR. Personally for fat burning, I'd think high zone 2 or low zone 3 ideal. Even on a bike, if out to burn fat, staying below my LTHR seems to do the best. I don't seem as ravenous after the ride. If I allow myself to go hard, then I feel as if I just eat back those Calories I burned off.