How long does it take to sail 250 mile in 6 knots of wind?

Depends on the boat, your choice of sail, the sea condition and how the boat is rigged. I'm going to say that, on a broad reach, with nearly calm following seas and a 150% decksweeper, clean fiberglass hull, sloop-rigged, I get I could get that done in 50 hours give or take a lot. Beating into that in a 35,000lb Wetsnail [sic] will take longer (and longer). Broad reachings on a 60' light air sled with a big chute up would take considerably less. Note that this isn't really simple algebra at all. Even headed dead downwind in a constant 6kt breeze, I'm not going to actually make 6kt SOG. I can think of 20 mitigating factors off the top of my head.