How long does it take you to design a story?

[SpiritSharD]

So after having begun my custom story, I've spent less and less time designing the levels. It's the same whenever I start doing anything creative; I'll work on it for hours at the start, but gradually spend less time until I eventually stop half-way through the project. A lack of motivation is what it comes down to, I think.

But I was interested in knowing; how long does it take you to design your custom story? What about each of the separate aspects of the design (scripting, maps, models)?

To compare; I'm currently finishing off work on my second map. I've done no scripting or models (don't plan on doing the latter either). I started my project on the 20th December.