Surprisingly common in fangames and mods. Mushroom Kingdom Fusion started in 2007 at the latest, and it's not going to be done for a year or so at the earliest. So, 20 years, maybe 30 if you count predecessor projects.

Brutal Mario started in 2006 at the latest, and ended in the 2020s as far as I recall, so about 14-15 years. Of course, the 2006 demo was technically demo 6, so we have no idea when the first 5 versions were created since they seem to be gone forever.

Super Mario Star World is a remake/sequel to the Mario 64 hack Super Mario Star Road, which was made for Super Mario 64 DS instead. It seemed to start out as a sequel in the Super Mario 64 engine in 2013, then ended up as a Super Mario 3D Land mod a few years later, got retooled as a Super Mario 64 DS hack a few years after that and is currently in development by a new creator after original creator SKELUX got cancer and moved onto other projects.

In other mediums, I guess there's the Thief & the Cobbler. Started in 1964, got an unfinished/messed up release in 1993, and a better recuts in 2013 and 2023.

"Few paintings have lasted the test of time as the great wave of Kanagawa. It took Kanagawa 40 years before he drew the wave he is most known for."

The painting is by Hokusai. Kanagawa is a place.

Software

Richard Stallman: GCC and Emacs - both massive pro?ects and still going strong.

Linus Torvalds: Linux OS kernel

Science

Geoffrey Hinton: scaling neural networks

Books

Donald E. Knuth: Structure and Interpretation of Computer Programs (vol 4c of 12)

Buildings

misc. architects and builders: (most) cathedrals (Cologne, Barcelona, ...)

I'm reminded of an old talk by Rich Hickey [1] (he created Closure, initially in semi-isolation over some years).

  > When was the last time that you thought about something for an
  > entire hour? ... Howabout for a whole day? ... or over the course of a
  > month? ... or a year? These are tremendously valuable moments, if you
  > get to have them at all.

Darwin took 21 years to publish his findings on evolution. In fact he couldn't finish the actual book that he had set out to write and published a summarized abstract instead, which is what we know as On the Origin of Species.

Myself, Roger Wagner,HyperStudio, 1988...1998...2008+

People who...

I guess Chris Roberts and Star Citizen will join that club

Ted Nelson!

A couple software examples off the top of my head are Richard Hipp with SQLite and Matthew Ahrens with ZFS. Both almost incomprehensibly sophisticated projects executed well over decades.

Neither are anything like a solo effort in the way these examples are, and it's hard to say what they'd be like without the long individual involvement.

In fantasy:

J R R Tolkien

Janny Wurts

M A R Barker

Ed Greenwood

the title kind of implies there would be more than just 2

Yes?

Growing up in a retirement community you would be foolish to ignore the fact that some things don't start getting good for the first few decades.

Most everybody else doesn't have such overwhelming examples, and fail to realize it until much later in life, if ever.