I agree with other answers that the speed to practice scales at is the speed where you can play them accurately, evenly, smoothly, and cleanly.
Beyond that, when you ask what the goal should be in terms of speed or where "diminishing returns" set in, I would counter by asking you, "Why are you practicing scales in the first place? What are you trying to get out of the exercise?"
I ask this for several reasons, but firstly because you state that you are trying to learn piano but have been unsuccessful several times in the past. Scale playing can quickly become tedious, and if you're trying to maintain your own interest as you're learning, I would suggest that you try to find exercises and types of practice (e.g., starting to learn actual pieces that you're interested in) that maintain your interest at the outset. If you find yourself getting bored and engaging in mindless practice, I think it's much more likely that this attempt to learn piano will fail like your previous ones.
In general, don't give up! Congratulations on sticking with your practice so far. It's a great goal to start trying to play piano again, so find exercises that will help you stick with it.
Beyond that, I ask why you are practicing scales because (this may be a controversial opinion) mindless practice of plain scales is pretty useless. (Before those who are reading this have a knee-jerk reaction, note that I have several qualifiers in that sentence, which I'll explain.) The main reason many pianists spend long hours practicing 4-octave plain scales is to pass exams, and my personal opinion is that that exercise likely collectively wastes millions of hours of practice time every year that most pianists could be devoting to other more productive exercises.
Note that I specify plain scales, by which I mean simply playing up and down the scale, often with no specific agenda, articulation, dynamics, or anything else other than smooth execution of the scale. Once you've absorbed the basic finger patterns to the scales, there's little reason to keep practicing simple ascending/descending scales over other patterns.
Many people will claim that scales teach you patterns that will be useful in reading actual music. That is true to an extent, but it's relatively rare to find long extended scales occurring all over a piece of music. And even when scales longer than a few notes do occur in repertoire, they are often modified in various ways (skipping notes, adding notes, played in parallel thirds or sixths or whatever) so that the simple specific ascending/descending pattern in octaves with both hands isn't that applicable.
Many people will also claim that scales teach you a bit of music theory. And that is also true when first learning keys, I suppose. But if you actually want to learn to play in different keys, then play music in different keys. I've met many pianists in my life who could perfectly execute a B major scale in four octaves without thinking, but they'd have great difficulty sight-reading in that key. Once learning the basic notes in the scale and the finger patterns, move on to playing arpeggios, chord progressions, and other patterns in each key.
Many people will claim that scales teach you skills to improvise in a key. Again, I say -- if you want to improvise in a key, then practice improvising in that key rather than simply playing scales. Learn all sorts of patterns and then try using them.
The other qualifier I made above is that mindless scale practice is useless. Practice should never be mindless. If you're "going on autopilot" when you're playing a scale, you're wasting time when you could actually be doing something musical. And that "doing something musical" could be as simple as things that Tim suggests, like using different articulations and dynamics. But once that gets tedious, begin to change the patterns. Practice arpeggios instead of scales. Do contrary motion scales instead of doing the hands in parallel. Do combinations of arpeggios and scales in each key. Then try playing various patterns rather than a straight scale. For example, start moving up a scale in thirds (1-3-2-4-3-5- etc.) or other intervals. Instead of playing in octaves with both hands, try other intervals (like thirds and sixths). There are many method books with exercises and etudes that do these sorts of things and provide many variations.
Mindless practicing is not teaching you to be a musician. So the moment you've gotten to a point playing an exercise that it's "mindless" and you're not able to pay constant attention because it's tedious, you might want to think of working on something else. If you haven't yet learned to do the exercise accurately and smoothly, come back to it later when you're ready to focus again. You've likely reached the "diminishing returns" mentioned in the question. (Even if you've reached the accuracy goal, come back to the exercise periodically and try it again when you've been away for a while. Hopefully by that point it will be less tedious, as you'll realize you don't have the fluency and need to actually pay attention as you practice to execute it well.)
Personally, I believe that learning, reading, and playing a large variety of repertoire is going to help you more in the long run than mindless repetition of scales. And it will probably hold your interest more than scales, if you've had trouble keeping to your practice regime in the past. Again, I'm not saying to neglect scales entirely -- but once you learn the basic finger patterns (and can play them evenly and accurately), starting finding ways to vary the exercises to keep building your skills.
Lastly, you ask what "experienced pianists" do, and I'd say it varies greatly. Some pianists love scales as warm-up exercises, just to get the fingers moving. You can do that, but other pianists just use some other lively piece of repertoire for the same purpose.
Other experienced pianists use scales for diagnostic purposes. That is, rather than playing them mindlessly, they will play them deliberately, paying close attention to the details of their execution to check for unevenness, improper technique, lack of fluid motion, etc. The benefit of a standard pattern is that it becomes a sort of "benchmark," where you can isolate the details of your technique from other musical elements specific to a particular piece. Scales are a common benchmarking exercise, used in a similar fashion to how an athlete might use stretches and warm-up exercises not only to "get ready" but also to note minor defects that need to be worked on more carefully. The scale is not the goal in and of itself, but it helps diagnose places where more effort is needed.
To go back to the original question's concerns -- those experienced pianists often aren't focused on a particular speed goal. Slow practice of scales might bring out different problems from fast practice for them. Use scales as a tool, not an end goal.