Depends on how long it is.
The William Boyd was about 300 pages, and an easy read. Maybe ten days.
Back in the 1990s, I read Marcel Proust's A La Recherche du Temps Perdu (3000 densely packed pages). That took me a full year.
But that's only the fiction. As I've said in this thread before, I always have several non-fiction books on the go. Because I only read a few pages at a time, they can take years (or in some cases, decades).
EDIT: I haven't yet decided whether to read the 12 book Powell series straight through, or to intersperse a few other things (which would obviously increase the lead time).