I can definetly relate to RMR. Albums that I really like, I tend to memorize from start to finish, from every piece of lyrics to every single note. It just happens. I have very good memory for sounds and music, but, like RMR, am rather forgetful when it comes to practical everyday stuff
When I'm on the buss or otherwise doing something while not mentally occupied, I tend to think about (or remember) music. As I'm a drummer, I'm often tapping my hands, fingers and feet to the music playing in my head.
edit :
As for how often .. Hm, this year has been a bit slow for me musically. I've listened to a lot, but I haven't explored so much new, so I've probably (fully) memorized three albums or so this year.