When I'm working on a side project, I sometimes only have a few hours a week to invest in it. It can take years for the project to be completed. If I want to have any hope of completing the project, I need to be able to plan for multiple years of slow and steady work.

This has gotten easier for me as I've aged (I turned 40 this summer). I'm not sure I could have sustained that level of focus in my early 20s.