For me it is deciding to do something.
That is when the project starts, the making it an actual thing to do.
Though it could be months before I actually buy anything physical, it could be lots of planning and reading first.
As for when it ends…?
So I have my new range coming out this year, 1 project, carrying on with expanding the existing ranges, 2 projects, and Celtos (still at the thinking/planning stage, with no purchases) 3 projects.