By that logic, everything takes a lifetime. That's not a useful metric.
No, those weekend projects really did take a weekend. They started without the project before the weekend, and they had the project after the weekend. That's a weekend project.
So imho, weekend projects are not necessarily wrapped up in a 2 day stint.
Little projects are a really nice distraction and I use them to clear my mind. I've found out early on that spending all my time developing the same thing, https://mijura.com in my case, was draining. Short projects also let me test out new libraries, hosting providers etc. That said, a weekend project can prove popular and then consume an immense amount of time. This happened to me with http://vodafail.com, which I initially created while waiting on hold for customer service.
I've always had the impression that, at least on HN, "weekend project" describes a level of "seriousness" rather than absolute time invested. It lets readers know what kind of scope and polish to expect when they click the link, and what sort of criticism would be helpful. Something like:
Startup > (Real Job) > Side Project > Weekend Project
If I click a weekend project post, I expect to see, e.g., a cute but not necessarily marketable idea, a clever technical hack that might not scale, or a cutting edge design that probably doesn't work in IE8. I expect feedback to focus on those things and not, say, funding advice.
ImageStash (since the article mentioned it ... yay, someone read my post!) took a good three months of weekends. But I thought of it as my Weekend Project because it was just something I started on a whim, and worked on sporadically when I was in the mood to code but burned out on both my Side Project and Real Job.
We built v0.1 of http://queued.at in 36 hours of coding at Dallas startup weekend. Certainly each of the people working on the project spent a LOT more time honing the skills used to create the application, but from the first lines of code to launch was only 36 hours.