I don't like spending too long on one thing.
Of course there's the risk of losing interest, but there's another factor that has hit me as well.
If I'm working on a scenario, and my skills/tools improve significantly before I finish it, I'll want to go back to the early stuff I worked on and remake it with newer better stuff. It'd end up just like the two guys who work full time painting the Golden Gate bridge - by the time they reach the end, the beginning needs another paint job.
It's sort of happening to me now on something that I haven't even worked on for all that long, but thankfully they stuff I'm going back to change won't take long to implement.
Obviously, the longer you can spend on it, the details will be more fine-tuned and polished, but I don't want to make the same scenario 3 times, and I also don't want to have a scenario with a funny mix of primitive and advanced systems.