To the average bowhunter I'd recommend 12" of fletch. While there are a lot of circumstances that come up where most any fletch will work, a 4" vane or feather will steer a fixed blade broadhead very well........5" even better. Feathers grab a little more air than vanes, but both work well.
Like B.H. says, the better your bow is tuned, the more you can get away with. Why chance it on a hunting shaft though? You get one shot with a bow most times.
Guys that shoot all year know exactly what they can get away with. The average bowhunter shoots a few months a year and , unfortunatly, mistakes or unforseen circumstances can arise when in the woods. Its not worth the couple grains of weight you save for less control.
There are vanes on the market like Blazers or Quick Spins that grab more air with a shorter vane. These are an exception to the rule and Blazers especially can control the shaft very well.
JMO,
Jerry