Let's start with quantifying language learning. From GlobalMaverick:
I don’t think it’s possible to say with any sort of accuracy that you’re at 73.6% of a native speaker’s abilities…And Language Fixation concurs:
[T]he distance between “beginner” and “native” is huge and unknown…I disagree with these statements, but I would be able to agree with them if they added "with the currently existing language-learning tools". A language is a limited body of knowledge, consisting of discrete units: vocab, grammatical rules, intonation patterns, etc. Once you've got a way to count these units (and all of them certainly can be counted), then it's simply a matter of figuring out what the average native speaker knows to be able to determine whether you're at 73.6% of a native speaker's abilities. An SRS system with each of these items broken down into a discrete units could do just that. I'd be surprised if a tool like this doesn't pop up soon enough, but until then, GlobalMaverick and Language Fixation are right; there's not a real accurate way to get the whole picture of where you are in a language.
Now let's get to diminishing returns. From Language Fixation:
For me there’s also a tremendous thrill at the beginning because everything is so new, and because every time you sit down to study, you’re actually gaining a rather large percentage increase in your knowledge. After day 2, you know TWICE AS MUCH as after day 1.This gets to a great point of debate in language learning. As you progress farther and farther into the language, finding those discrete units that you don't already know takes more and more time. Thus, going from 90% to 99% proficiency will take a heckuva lot longer than going from 0% to 10%, or probably even 0% to 50%. The point of debate this brings up is whether it's better to get a bunch of languages up to 90% proficiency, or one or two up to 99% proficiency.
And that gets us to linguistic wanderlust, i.e., the desire to work on getting a new language up to 90% before the previous one is up to 99%. As Language Fixation describes, the speed at which you can do this always make it attractive to the serial language learner.
I've had to restrain myself from indulging my own wanderlust (despite frequent flirtations with Russian, Korean, and Arabic) and focus on the seven languages in which I've already got some degree of proficiency in, and all of which need a lot more work, whether on the way to 90% or 99%. I suppose this puts me in the aim-for-99% camp, but we'll see if I can maintain my self-control (I'm pretty sure the next time I see a good Korean grammar book in Japanese, I'm going to buy it).
Link: the size of a language [Language Fixation]
The messy in-between [GlobalMaverick]