I think the answer to this question varies greatly. That might be why no one has responded so far. Just as a quick comment, it depends on the objective of the working demo.
A demo, could be a storefront showing fictitious products, that you can’t really buy yet. On the other hand, a demo could be static, non-functioning, Photoshop pages, looking more like a storyboard.
Then in your question you added the term “working” to the word demo. A demo is generally a demonstration which may not be working. When you get a “demo” of a new mop, the mop is generally working… the demo is how to use it.
When you are looking at websites, demos usually don’t work. If the demo worked, the job would be done. Another example is, I draw a picture of a car or show you a non-drivable automobile. Then we decide to build a working model. The working model is likely still considered a demo, but the cost for this demo VS the cost of the drawing demo is significantly different.
So, if you are looking for a “working demo” it could be several weeks. Because after all, the website is working. But if you are looking for someone to design a look and a feel, but nothing can be bought. Or a look and a feel, but really nothing can be browsed through as a real website, then only a few days.
Without getting too involved, I hope this helps.
Vincent Wilcox (a.k.a. KRAKR)
My band: Letters Make Words