Renowned computer expert Donald Knuth began work on The Art of Computer Programming in 1962, expecting to finish later that year. His publisher convinced him to divide the book into seven volumes, the first of which was published in 1968, six years later. Two more volumes followed in '69 and '73. The fourth volume proved to be so lengthy as to necessitate being further split into a projected three subvolumes, the first of which, 4A, was released in 2011. For those of you keeping score, that's a 38 year schedule slip, in a project that has been ongoing for nearly half a century.
Contributory to schedule slip after volume 3 was Knuth becoming so irritated with the progressively worse typesetting each edition had had that he decided to create an entire typesetting system (TeX). As above, he seriously underestimated delivery time. He expected to finish it within a year. Seven years later, the software and multiple volumes of supporting documentation were completed. (This is a not unfamiliar problem with computing people known as Toolsmithing: Getting completely sidetracked from the task at hand by trying to develop the proper tools for the task at hand.)
A semi-famous Sports Statistics page (Doug Stats) has had this happen because the sole webmaster works fifty hour workweeks at a different job, and also happened to have been raising a family on top of that. He does regularly get stat upgrades, but when things become severe at the start of the season, people start complaining.
Priest. The first ten volumes were released within a few months of each other, then the gap between volume releases got longer and longer, and now it's been three years without any sign of Volume 17.
Dilbert: Though never happening with the comic itself, this trope is invoked with the 'Dilbert Newsletter' readers can sign up for, which author Scott Adams describes as 'coming out approximately whenever I feel like it'.
The electronic band Ego Likeness intended to release their Compass EPs over four years, starting with South in 2007. However, East was not actually released until 2012.