The question is whether taking a regular break makes the hours you practice more effective.
Wasn't the original question: "Do any of you have a feeling that you make better progress if you take a day off at regular intervals?"
I accidentally deleted a few important sentences in my previous post (I tend to ramble so I end up removing at least half of my original comments).
I originally wrote something along the lines of: Person B would need to be about 15% more efficient to progress as well as Person A
My answer to your new question: without a doubt, yes. Standing up, stretching, and walking around for a minute or so every 30-40 minutes makes my practice much more efficient. I don't know why, but I tend to have better focus if I take these mini breaks.
If you're talking about taking a break once a week, I doubt the break will make the time you spend practicing more effective. For me, effective practice is all about staying focused. Perhaps that one day will do wonders for you; however, when I come back from a day or so break from practicing, the passages I've worked on are pretty much exactly how I've left them. Sometimes I may have a revelation away from the piano, but it's rare.
I pretty much never feel unmotivated; I always enjoy practicing.
You are blessed. I always hate practicing mainly because I go through multiple love-hate cycles with the pieces I learn. When I start practicing I usually hate my playing and ultimately the piece. Eventually I achieve the sound/ideas I want and come to like the piece once again. Finally I go to my lesson to sometimes have my ideas absolutely destroyed by my teacher convincing me to hate my playing once again.
But sometimes when a trip or something else keeps me from practicing for a day or two, I feel like I've continued to improve, and that the subsequent practice has been more productive.
I find this hard to believe.