I read Feist's Faerie Tale in March of 1991. I know what book I was reading 25 years ago. I know what book I was reading the day I turned 18, and how many books I read in 1983. (The White Dragon by Anne McCaffrey; The Return of the King; 123.) I know these things because while I am relatively cluttered and disorganized in most of my life, in at least one area I am severely afflicted with OCD.
As best as I can recall (this was a while ago), some time in May of 1982, near the end of my sophomore year in high school, I was for some reason trying to remember the last handful of books I'd finished, and so made a list. Then I added the next book I read, and the one after that, and after that, and etc. And since it took like two seconds to make a short notation whenever I'd finish a new book, I kept doing it.
I did fall out of the habit for 4 years, from 1986 through 1990 (girlfriend; little interesting in jotting down quick things; I'm sure you understand), before picking it up again; and but for a few instances of forgetting exactly when I'd read something, have soldiered on ever since.
Why? Well...why not? I love lists of things; I find inordinate pleasure in certain kinds of order. So why not a list about my reading? Like I said, it takes like two seconds to add to the list once ever week or so, so it's hardly some arduous task I've embarked upon. And it's extremely interesting to go back through it once in a while to remind myself what I liked to read when I was a teenager.
How long will I continue it? I have no idea. Maybe forever. Maybe only as long as I like to read.
So, like I said...maybe forever.