*evil grin* However long you want it to be!
Alright, all kidding aside, that kinda sorta IS the actual answer. But I suspect what we should really discuss is how long should a scarf be for any given style. So let's break it down into some categories!
To me, a micro scarf is an accessory that is really just draped around the back of your neck and pinned (or stitched) very close to the bottom of the clavicle. It's a great length to wear over a long-sleeved t-shirt or tucked into the top of a zipped up jacket. It keeps your neck warm and doesn't offer much to fuss over in the the way of pieces hanging to your waist.
A great example of this is the Maxi Wool Rib Muffler by Erika Knight. Many of these types of scarves are 1-skein projects so they knit up quickly. This little cutie in the sketch is 18" (46 cm) long and 12.5" (32 cm) wide.
On average, these little neck-warming scarves are between 18" and 30" long (45 cm to 75 cm) when laid flat and can range from just 4" (10 cm) tall/wide up to 15" (38 cm).
HALF WRAP SCARVES
Keeping in mind these are my designations... to me, a "half wrap" scarf ranges between 30" and 60" (76 cm and 153 cm). Often times these are worn with one long tail of the scarf down the front of the body, and the other half thrown around the neck and left to hang down the back (as in the image below). These are one of the most common lengths of scarf. As you get closer to the 60" (153 cm) length, you can wrap both ends of the scarf around your neck and have both tails hanging in front, but it will be wrapped very close to the neck and the tails will be short.
A fabulous version of a half-wrap scarf that you could wrap all the way around is the Yabane Scarf by Olgajazzy. This delightfully architectural scarfs is designed to be knit to 55" (140 cm) so it can be worn a multitude of ways.
FULL / MULTI-WRAP SCARVES
This final self-made category is for the scarf-maker who just doesn't want to quit! Knit to staggering lengths for major "wrappage" - i.e.: wrapped multiple times around the neck - these full/multi-wrap scarves often exceed 72" (183 cm) in length and are more often than not only knit to a width of about 6" (15 cm), though exceptions certainly apply!
As an example, the Kauni Humboldt scarf is knit to 80" (203 cm) and can easily be wrapped twice around the neck. Soooooo cozy! If it were to be wrapped only 1 time, the ends of the scarf would likely hang below the waist.
So! Pick your perfect length and get to knittin', kittens!