Knitting a scarf is usually the first project a new knitter will try after getting the basics down, and it's often the first kind of project that someone tries to design on her or his own. Scarves are easy to design and knit because they are usually just big rectangles, but the question often then becomes: how big should that rectangle be?
The short answer is: as long as you like. There are some scarves, like the Razor Shell Scarflette, that are really short but are also effective at that length (in the case of that one, it's cashmere, so it's warm even though it's short and you don't want to spend the money to knit a five foot long scarf in cashmere).
And some scarves, like the famous Doctor Who scarf, are longer than could be considered practical to the knitter or the wearer.
In general, however, a scarf needs to be long enough to wrap once around the neck and have some length left over to tuck into a coat of leave dangling as desired.
I remember once hearing the tip, possibly from Vickie Howell but it was so long ago I don't really remember, that a good length for a warm scarf is roughly the same as the height of the recipient. I try to use this rule of thumb generally when I'm designing warm scarves and shoot for around six feet in length (I'm not six feet tall, but it's a nicer number than five foot six).