It really depends on how you use your TaskPaper file … I don’t think there’s any particular maximum. It used to be that you would have to see all the indentation (in TaskPaper 2) even when you focused into a project. But in TaskPaper 3 when you focus into a project you just see that projects indentation … so there’s no much cost to indenting projects as long as you edit them in focused mode.
I would say my own lists don’t go much beyond 7 levels. But I’m sure there’s someone out there who creates really deep hiearchies!
If you want to share it I think best to release in your own repo and then link to it from the TaskPaper extensions wiki. I don’t want to build up to many official examples right now.