Does a run of 1000 feet of CAT6 line need to be in conduit?
To directly answer your question, there is direct bury ethernet cable. You would probably want to go at least 18&quot; deep with it. But there is still a chance it will break, and then you'll be replacing the whole run, so it might make sense to place it in conduit right away to make future replacement easier.
However, the real issue is that 1000 ft is much too long for Ethernet to work properly. For CAT6 cable, the general limit is about 300 ft, which can probably pushed a little, but not 3x as long. In your case, the best best would be to either run fiber optic cable, coax, or use a wireless bridge. There are some reasonable options available for either of those. Fiber is not trivial to install unless you get pre-terminated cable. For either fiber or coax, you would need some sort of converter at either end. The wireless link would probably be easier. Wireless would also require devices at each end to convert wired/wireless. All options would require electricity at the end points.