I believe the 6 feet limit on flexible metal conduit (which enters most people's minds) is not the maximum length allowed for the raceway, but rather is the maximum length it can be run unsupported. This gives many of us an assumed general rule that the max length in all cases is 6 feet.
You also have a maximum length of 6 feet if the flex is used for the equipment grounding conductor to throw into the confusion, but most pre-made "whips" have an EGC installed in them and really is not an issue with those either.