When I got a permit to run 220V underground, I was required to go at least 18" deep.
I would follow that recommendation for your situation as well. In addition to the pull wire, I would pull several extra network (copper and fiber) and coax wires. It's a lot easier to pull all the wires in one bundle than to pull them one at a time. Make sure the wires are all water proof.
After filling the trench about 80%, put a plastic tape in the trench. The tape is bright colored and contains writing along the lines of 'buried cable'. Then finish backfilling.
Document the location of the trench and of course the contents of the conduit.
Call the locator service to make sure you don't cut other wires/pipes when digging.
It against wiring code to pull "high voltage" wires (110V or 220V) along with "low voltage" wires.