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.