It entirely depends what it is. A Liszt Rhapsody (no 6 is the most recent) took me 2 hours to learn from memory and be able to play (despite confusing some of the inversions in the slow part). There is very little fingering to learn, and most of it is repetition (transposed), so the brain does not have anything complicated to swallow. However, I (personally) find learning (even to play from the score) anything fugal tremendously arduous, and I have to work very hard to swallow a 3-part fugue of bach. Generally, I find that so long as I can easily identify the chord and inversion, I can memorize extremely quickly. For study of a new work, I generally play through, slowly practice the odd bar that I cannot identify the pattern or harmony of, and just let the notes settle in to my system over a week or so. For instance, the Liszt Sonata has taken me a week to learn and master the mechanical problems of (2 blasted bars of octaves at the end still elude me however!) on about 2 hours a day. for something like this, I would not try to memorise it, just continue to practice it and the memory will come soon (touch wood!...). The first movement of the Appassionata I learnt in 2 hours, and the last movement in 2 days. But this method of learning is mostly based on a sound knowledge of scales and arpeggios (and regular sight reading and improvisation), so that as soon as I see a new pattern of fast notes for example, I can instantly identify the fingering and hand postion. So, the runs in chopin scherzo no2 would be doable at sight (the pattern is common), but the passages at the end of Rachmaninoff sonata no.2 would take time just to absorb the fingering and get my brain acquainted with the pattern as it is not that common. I hope I wont get a chain of posts criticizing my awful analogies, im tired and its the first thing that springs to mind.