How can you tell if you are making progress fast enough?

Obviously people learn at vastly different speeds and so on, but I still think there must be a good idea of whether you are learning something too slowly based on, maybe an average of how long it takes someone to learn a piece?

I don't know where you could find this information except maybe by talking to teachers or students. For example, how many years must someone be playing on average to learn Bach's C Minor Prelude from the Well-Tempered Clavier book 1?

The reason why this is important to me is because I want an idea if my practice habits are bad or if I'm moving along just fine (or even splendid!). I remember hearing there is a book out there of piano repertoire rated by difficulty or something, but I can't remember the name.

I always grade myself, "capable of improvement." Nothing more, nothing less. In that way, I strive to improve everything I do at all times.

There are lots of books of music for different grades, published by the ABRSM and others. So, in general, if you can play a piece well in one grade you should be able to manage all the others, though you'll still find that some take longer to learn, and some seem more difficult - that's because we all have different technical abilities and find some things easier than others. If you don't have a teacher, you'll only find out by trial and error what standard you are at.

As to how long it should take to learn any particular piece - I'd say: as long as it takes. Obviously, if you hit a brick wall very quickly, the piece may be too far beyond your present technical ability, and maybe you should set your sights lower for the present until your technique catches up with your ambitions. When I was young, I (somewhat rashly) treated myself to volumes of Beethoven Sonatas, and decided I was going to start learning them, though I was only Grade 6 at the time and had only played a couple of slow movements of the Sonatas. But it did mean I had a goal to aim for in the next few years.....

I listen to myself intensely, and think - "so how shitty is this right now, compared to a few days/weeks ago?"

And therein lies your answer.

You may be thinking of Jane McGrath's Pianist's Guide to Standard Teaching and Performance Literature. That's a very helpful book.