Long enough so the hat can be dropped down onto the upper back. When the sun comes out the neck is protected from being burned. While wearing a Seattle Sombrero you can hear normally, move your head normally, wear glasses comfortably, and vent away heat much more efficiently. Contributing greatly to the popularity of this hat is the micro-climate of relative dryness that is created around the face and neck of the wearer. If you are an eyeglass wearer this can be very important.
Photographers also report greater comfort in working with cameras in the rain.
The Seattle Sombrero works well as an awning for a camera on a tripod in the rain. Also very important for comfort is the fact that it is made entirely of quick drying fabrics. A few minutes after a nasty rain storm is over the hat will be dry.