I was playing a Bach fugue (originally learned in 1955 - or that is what the dedication from my piano teacher says in this somewhat tattered book). The first 20 or so measures were still there in muscle memory but that faded pretty fast. I used to know the whole piece. Then I tried sight reading at about 1/2 speed and it was tough; I never was very good at sight reading. My fading repertoire includes several Schumann kinderschenen, a Beethoven bagatelle, and a few pieces of Bartok for Children. I didn't get to the Haydn sonata.
Of course I only try this when
rsc isn't here - noone should have to hear me. I always resolve to practice some more so that I can play at least acceptably to me, but it is pretty rare that I actually do it. 