How deep should liquidtight flex conduit be burried?
356.10 Uses Permitted. LFNC shall be permitted to be used in exposed or concealed locations for the following purposes:
Informational Note: Extreme cold may cause some types of nonmetallic conduits to become brittle and therefore more sus-ceptible 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 suit-able for the purpose.
(4) For direct burial where listed and marked for the purpose.

Notice number 4.
Here is an excerpt from the UL Whitebook concerning LFNC

Flexible Nonmetallic Conduit, Liquid-tight (DXOQ) USE AND INSTALLATION
This category covers liquid-tight flexible nonmetallic conduit, in trade sizes 3/8 in. to 4 (metric designators 12 to 103) inclusive, for installation in accordance with Article 356 of ANSI/NFPA 70, ‘‘National Electrical Code'' (NEC).
This product may also be used for installation of conductors for electric signs and outline lighting in accordance with the NEC.
PRODUCT MARKINGS Liquid-tight flexible nonmetallic conduit suitable for direct burial and in poured concrete is marked ‘‘Direct Burial,'' ‘‘ Burial,'' ‘‘Dir Burial'' or ‘‘Dir Bur.'' Liquid-tight flexible nonmetallic conduit suitable for use outdoors is marked ‘‘Outdoor.''

According to Table 300.5 column 3 you would need 18&quot; of cover. If you have a 120volt circuit that is GFCI protected you can reduce the cover to 12&quot;.
Notice the table reads &quot;minimum cover requirements&quot; NOT burial depth. With small raceways this is insignificant but larger raceway should be measured to the top of the raceway not the depth of the trench.
[Note: I had previously cited Article 350 which is LFMC not LFNC and referred to manufacturers information for burial depth which is actually covered by Table 300.5 of the code. Upon re-reading the Code I edited this answer accordingly adding the citation to the UL Whitebook. Hopefully this clarifies some things.]
Good luck and stay safe!