I haven't researched the issue, but my brother did several years ago, and he found that for walking/running the number of calories were more impacted by distance than time.
It makes sense that it would be transferable to cycling as well. So, ideally, I think that riding as far as you can in the time you have available to cycle, and most importantly watch intake of food.
I like focusing on cardiovascular health when riding, and the calories burned follow along. I ride most of my rides at a steady pace, but throw in some short times of extra exertion to build up my stamina. Sometimes this means just riding a little above my comfort level for a mile or so. But Saturday for the first time in many years, I felt comfortable enough to get out of the saddle and sprint a couple of times. Nothing can stop me now.
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Slow Ride Cyclists of NEPA
People do not seem to realize that their opinion of the world is also a confession of character.
- Ralph Waldo Emerson