It's always a question of how long since the last time I was reading a book before it moves from "currently reading" to "abandoned".
I have a couple ("Dhalgren" & "The Windup Girl") that have been parked for about a year now. I do plan to finish them eventually.
I have others that I've read sometime in the last two months ("Stand on Zanzibar", "The Dark Forest", "Jonathon Strange & Mr. Norrell").
It's worse if I consider technical non-fiction books.
@TerryHancock Dhalgren and Windup Girl are both so good!