For me personally I mod when I feel a need to mod. For Morrowind and Oblivion this resulted in one big project that I just had to do and where I was asking (too many *lol*) favors of modding friends to help me and several smaller ones that were just for fun, to test certain theories or to make something in particular for someone else. Next to that I enjoyed helping out other modders with playtesting, voiceovers or offering suggestions for their projects (provided the projects interested me or I knew the modder involved in its creation)

My first mod will probably happen after playing the game for at least several days as I want to immerse myself into the world first, and depending on my mood and my desire to change something when I feel like I simply have to change it. For Skyrim it could be something trivial as renaming Dwarven->Dwemer and Elven->Ayleid for weapons and armor, or setting up a nice cozy home if those already ingame are not to my liking.
I doubt I'll undertake massive projects again like I did with the Playable Dremora (Morrowind) or Mannimarco Revisited (Oblivion); but you never know

Greetz,
Milt