There is lots of mention of 6' in flexible conduits, allowing and dis-allowing in different ways, and all kinds of exceptions, that are easily confusing.
350.60, 356.60 both require a ground for equipment where flexibility is required. Also see 250.118(6,7+8)which limit ground path to 6 feet. But only with metal flex...
Sealtight? Metal? Carflex (Carlon) is LFNC, plastic, no ground path.
356.10 Uses Permitted.
LFNC shall be permitted to be used in exposed or concealed locations for the following purposes:
FPN:Extreme cold may cause some types of nonmetallic conduits to become brittle and therefore more susceptible to damage from physical contact.
(1) Where flexibility is required for installation, operation, or maintenance
(2) Where protection of the contained conductors is required from vapors, liquids, or solids
(3) For outdoor locations where listed and marked as suitable for the purpose
(4) For direct burial where listed and marked for the purpose
(5) Type LFNC-B shall be permitted to be installed in lengths longer than 1.8 m (6 ft) where secured in accordance with 356.30
(6) Type LFNC-B as a listed manufactured prewired assembly, metric designator 16 through 27 (trade size 1/2 through 1) conduit
356.12 Uses Not Permitted.
LFNC shall not be used as follows:
(1) Where subject to physical damage
(2) Where any combination of ambient and conductor temperatures is in excess of that for which the LFNC is approved
(3) In lengths longer than 1.8 m (6 ft), except as permitted by 356.10(5) or where a longer length is approved as essential for a required degree of flexibility
(4) Where voltage of the contained conductors is in excess of 600 volts, nominal
356.30 Securing and Supporting.
Type LFNC-B shall be securely fastened and supported in accordance with one of the following:
(1) The conduit shall be securely fastened at intervals not exceeding 900 mm (3 ft) and within 300 mm (12 in.) on each side of every outlet box, junction box, cabinet, or fitting.
(2) Securing or supporting of the conduit shall not be required where it is fished, installed in lengths not exceeding 900 mm (3 ft) at terminals where flexibility is required, or installed in lengths not exceeding 1.8 m (6 ft) from a luminaire (fixture) terminal connection for tap conductors to luminaires (lighting fixtures) permitted in 410.67(C).
(3) Horizontal runs of LFNC supported by openings through framing members at intervals not exceeding 900 mm (3 ft) and securely fastened within 300 mm (12 in.) of termination points shall be permitted.
Long story short, longer than 6', it needs to be supported at 12", and every 36".