I tend to set more life-long projects (photographing along Prairie Creek, for example) and they tend to be fairly loose.
I have a project along Redwood Creek that I recently started, but it also has no end-point, but hopefully "done" before I can no longer backpack (with a 4x5 camera).
I had a project that was basically a 6 month bicycle tour in New Zealand (1986/87) that was self-limiting since I could/can not afford to fly over there to flesh it out.
That is pretty much the limit of my projects, though I guess photographing my boys in the landscape is another project that is limited by my boys growing up and being willing to pose for me (they are 16 years old now).
Three Boys, Three Snags, 2008
8x10 negative, carbon print