Okay, some more unscientifically collected information ("anecdata"?), and then I should do useful things:
As noted above, when I tried to recall a piece of music that I haven't heard or played in months, if not a year or two, I was off by a quarter-tone (or slightly more).
However, just now, I tested myself with two pieces that I have been working with in the past week: the Queen of the Night's "Der Hölle Rache" (from Mozart's [i]The magic flute[/i], and "Isolde's transfiguration" (from Wagner's [i]Tristan and Isolde[/i]). Recalling the former's [i]altissimo[/i] arpeggios in F major, I was able to find an F exactly! Likewise, recalling the latter's opening fourth, I was able to find Eb and Ab exactly. (At first, I thought I was way off, because I was thinking that it starts with F# to B natural... then I remembered the correct starting pitches, and realized that my aural memory was dead-on.)
Lastly: I first realized that I could retain pitches from day to day when I was taking voice lessons about 15 years ago. One of the selections became my standby for finding B to C (Tom Rakewell's recitative "Here I stand," from Stravinsky's opera [i]The rake's progress[/i].) Even five years after those lessons, my memory was still reliable for this. After the Mozart and the Wagner, I decided to stretch by finding Tom's opening notes... but I was off by several semitones.
The moral of the story: My memory for pitch is most reliable over a span of hours, or days. Months and years, not so much.
P.S. Just to clarify the experimental conditions: I wasn't practicing or listening to music between posts (au contraire, I was avoiding work by playing poker on FaceBook).