Lee, like Bill Clinton said, "It depends on what your definition of "some time" is. Unless we have played our songs so often that it has sunk into muscle memory, then it will fade sooner than we would like. When I have a song memorized into my finger muscle memory, my mind can actually go into highway hypnosis, where my mind can drift off, and my fingers are still playing accurately, and I can't believe my finger are playing without me for a stretch. Of course, such episodes defeat one of my loves of music, which is to use it for active meditation. However, like meditation, the monkey mind has to be perpetually guided back to centered focus. My learning songs by ear, involves a lot of trial and error (“In search of the lost chord”) so I think I would benefit from learning songs by sheets. It's all about reinforcing learning and memory. Learning to read music better, is on my bucket list. In answer to, "Can anyone learn to play by ear?" I don't think so. We all know some people that are "tone deaf" and cannot sing or whistle a tune. Some people cannot discriminate pitches well. That's certainly a challenge if you want to be a musician.