For top speeds, that's easy. Hull speed is a practical limitation of how fast a given displacement hull can move through flat water, and depends solely on waterline length (note that this is the sailing waterline, which may be greater than the displacement waterline as heeling places more of the hull in the water).

In particular, the hull speed in knots is approximately 1.34 times the square root of the waterline length in feet.

For a generic tall ship of, say, 150 feet, this formula gives a practical speed limit of 16.4 knots. 18,000 miles is about 15,600 nm, which corresponds to about 40 days of travel if maintaining top speed continuously. This is our lower bound - in reality, varying conditions, storms, high pressure systems with no wind (doldrums), and inefficiencies in the necessary route will cause the actual velocity made good (closing speed to the destination) to be lower.

Here's a good record for you.

In 1851, the 225 foot long clipper ship Flying Cloud set the record for the fastest journey from New York to San Francisco, 16,000 nautical miles in 89 days. As Wikipedia mentions, the average ship during that era took on average around 200 days.

Hull speed has nothing to do with wind direction.

It just gives the maximum speed for displacement hulls independent of what drives them.

Of course there are ways to go faster, like planing and "cutting through" the bow wave (like most catamaran designs and other semi-displacement hulls do), but both of those are out of the question for tall ships.

And even for smaller hulls, those rules are interdependent of wind direction.

Small cats will enter the semi-displacement regime going upwind, and performance skiffs will easily plane upwind.