I gave it a serious try, reading this on and off for two years.

Tietjen auf Tour: Warum Camping mich glücklich macht (Why Camping Makes Me Happy)
by Bettina Tietjen
DNF at 150 pages (about 55%). Well, it was more like 84 pages of actual reading and then some skimming.
Travelogue for campers. Nice anecdotes, well written, but pretty repetitive. Each chapter felt the same after a while.
Reiseanekdoten für Wohnmobilfans. Nett, gut geschrieben, aber nach einer Weile fühlten sich alle Kapitel gleich an.
When do I DNF a book? I tend to give it at least 50 to 100 pages, depending on the size of book. 20 to 25% for ebooks. And in some cases I might decide to skim very hard to get an impression of the rest of the book.
How about you?