This is a good question but a bit of an unfair one: I agree totally it's quality not quantity. I play several instruments but only take lessons on 4 of them. With my violin studies I spend a long time on one piece (at the moment it's the Tchaikovsky concerto, which is taking me a very long time) as this is my principal instrument, and I believe in getting things as near perfect as I possibly can (I still have a long way to go, but I'm really enjoying it!)
With piano I get through pieces a lot quicker, but I am not working towards anything specific at the moment - I am doing a lot of work on my major work at the moment (Mozart's Piano Concerto K488 in A major) but am also learning other pieces, such as sonatas, fantasias etc., to add a bit of variation into my playing. I also like to buy pieces and sightread them - these will generally be easier pieces.
With singing I usually sight-read a new piece every week, but am currently working on some Scarlatti and some Kurt Weill.
With the French horn I don't have a teacher at the moment, and haven't had a teacher since February, but have been buying more music anyway until I can find someone new.
With the viola, recorder, guitar and organ, I teach myself.
So, were I to perform absolutely everything I've "learned" this year, it would be a pretty third-rate recital, with some occasional good stuff that I've actually worked on.
What about orchestral pieces? Do they count? Because I spend a lot of time looking over violin parts!