Not sure there is a correct answer to this but I curious to hear your thoughts and arguments.

Maybe a better question might be what amount of studying would result in the most amount of characters being retained with the least amount of study time.

If you studied only a few characters each day, then it would take seemingly forever to learn all of them, even if you had 100% retention. Conversely, if you studied too many, you would retain too few, so you might be stuck in a cycle of trying to learn the same characters everyday, using up lots of time, but never actually remembering them.

In my own case, I find when I get up to 1-1.5 hours of time as recorded by Skritter each day, that is optimal. Beyond that, I end up forgetting too many characters, so I end up retaining less than if I had spent less time learning fewer characters each day.

It's hard to answer because it depends on what else you're doing with the words. If you are learning from a textbook and using those words every day you can learn far faster than if you are studying words out of context.

I always had the impression that high retention rate meant you learn less new characters/words, but the ones you do learn, you are less likely to forget. Low retention allows you to have exposure to more words/characters but your more likely to forget them.
Therefore if you are under pressure to learn a lot of characters then a lower retention rate would be better, especially if you are getting exposure in other ways. Likewise if your not in a big rush you can have a higher retention rate and learn all the characters better. Personally I have the highest retention rate because I don't have a pressure to learn characters, but if I had lots of characters to learn for class I might have to lower this in the future.