three (two John Barnes books and one of rezendi's) I read between zero and two days before buying them, then bought in order to encourage the author to write more.
10% I read within one day of acquisition
20% I read within two days of acquisition
50% I read within two weeks of acquisition
75% I read within three months of acquisition
90% I read within thirteen months of acquisition
The longest gap so far is just under six years, including four house-moves, between borrowing Midnight's Children and finishing it; on the other hand, there are 142 books that I've recorded buying and haven't recorded reading, of which the oldest is a copy of Nevil Shute's On the Beach that I picked up in April 2001 in the States.
I think a person more averse to clutter than I might contemplate donating to a second second-hand bookshop the set of books picked up in second-hand bookshops more than two years ago and not yet read. I've read only the lower-brow half of the intimidating pile I bought at Eastercon 2006.