When you just had enough of your job and want to move on, you would normally turn in your two-weeks notice. What is this 2-weeks anyways? It's barely enough time for your boss to hire your replacement, let alone enough time for you to train him. And it's definitely not enough time to finish your long-outstanding projects. So why 2 weeks, not 3 weeks? Or even 1 week for that matter since nothing is going to get done anyways? The 2-week notice is adopted for the following reasons:

1. It's short enough time to avoid your boss from dumping any additional projects on you (Uh, sorry Boss, no can do!)
2. It's short enough time to create panic and chaos in the office if they can't replace you quickly enough (now, they know what they're missing!)
3. It's short enough time to express the real reason behind your leave without worrying about your upcoming performance review (I hate you, and I hate this job!!)

and...

4. It's long enough time to retrieve any successful projects you've completed for your future portfolio (Yep, I did that!)
5. It's long enough time to clean out your personal junk on your computer (get rid of that Messenger program quick!)
6. It's long enough time to enjoy the farewell lunches and parties your co-workers throw for you ("For he's a jolly good fellow..." no singing plz!)