Learning guitar - How long does it take?

How long does it take a complete beginner to learn to play a guitar properly?
My pat-ready answer is 'not very long'.  For a more detailed one, it depends on what you mean by 'properly'. 
 There are a few stages of guitar learning that everyone goes through and each one of these relate to different levels of skill. Certain things you will learn quickly and others will take longer. For example, if your final destination is the ability to strum sing-a-long songs on the beach or around the fire you'll reach your goal faster than the guy who wants to play speedy metal solo's.
Here's a breakdown of the stages of guitar learning and the approximate time it might take you to get there.
Playing Basic Chords - This is usually the first benchmark most new players reach:  The ability to strum and switch between the basic guitar chords.  At this point you don't necessarily have the chords memorized and aren't able to play many songs, but you're managing to fret and strum them.  This level can be reached within one week.
Playing easy songs - You've mastered more guitar chords and have been practicing them enough that you've got most of them memorized and are able to strum and switch between them without much hassle, and you can even take your eyes of the fretboard when doing this.  This means you can play songs!  You can reach this level n two to three weeks of daily practice, though for many it might be longer.
Playing Barre and Power Chords - Moving right along, you're advancing to a level where most self-taught guitarists never go.  Barre Chords are much more difficult than open (or standard) chords and learning these takes extra practice and a different kind of strength in your fretting hand.  It is possible to play most barre chords in two months, quicker for some who practice regularly.
Fingerpicking - The ability to pluck individual strings and play simple riffs (short solo pieces on individual strings) is a new skill your strumming hand needs to learn.  This can usually be achieved in two to three months if you start to learn and play easy guitar tabs.
Lead Guitar - A Lead guitarist is the guy or girl who's got the audience at his/her beckoning.  Playing solo's and using techniques like slurring, vibrato and palm muting is a whole new set of rules and I'd give it three to four months.  This might be way off depending on what kind of music we're talking about - playing acoustic solo's might take less time, but playing rock or metal solo's will probably take longer.
The improviser - A guitarist that can improvise riffs and solos on the spot (and in the middle of a song), have moved from the realm of amateurship to being a pro or semi-pro.  Depending on how much talent you have and what you're musical background is, this might take anything from six months to a year.

Please note that these time lengths are given as a very broad estimate and aimed at people with little musical experience.  If it takes you longer than a week to learn the basic chords, don't get upset.  These times are not set in stone and you have to learn and develop at a pace that's natural and comfortable for you!