We have documented evidence in the form of podcasts that it took a team of 3 talented developers about 6 months to build StackOverflow. If you're looking for an order of magnitude estimate for how long it would take to reproduce it, that's it.
How exactly are you planning to reduce an 18 man-month project down to a single weekend of your time?
More generally, why is this attitude so common among programmers? How, in the face of documented proof to the contrary could an intelligent person like the parent still consider a site of the complexity of StackOverflow to be a "weekend job"?
The boil it down to a cliché, copying what someone else has built is exactly the same problem as rewriting an existing application from scratch, without the benefit of being able to read the source code.
So, yeah, be wary of "hey, that looks easy!", but don't think things are impossible either.
Also, with something like SO - you can pretty much see the models, and how they interact. That's already giving you a lot.
"Shit's easy syndrome" http://steve-yegge.blogspot.com/2009_04_01_archive.html