www.handgunlaw.us Is a good resource for both CCW and
knife laws in the various states. Actually 3.5' seems to be the most common limit. Also, the more vague issue is whether the knife is concealed or visible in some states.
There are a lot of gray areas as to how legislation would be interpreted in many instances. That being said with asking a few questions, doing a bit of research and using common sense a person should be fine in most cases.
The most important thing is to obtain quality training With drones (dedicated training knives with no edge or point but made to mimic an actual design). This allows real sparing and blade contact dynamics to be learned. Even a 3' knife is very formidable! With training a knife might just save a persons bacon or at least a goodly portion of it.
Thanks! This will allow people to carry the biggest most capable weapon within the limits of the law of each location.