The distance from Shanghai to San Francisco is 5398 nautical miles. At 15 knots, it takes 15 days. If you use a different speed, you do the math.
it all depends on the boat's engine and how fast you are going ..
What type of boat? How fast? From and to where? Last time I went from Hong Kong to Tacoma, WA it took about 9 days. We had a 9 cylinder diesel engine that I could crawl inside of. It was 3 decks tall and put out around 18,000 hp.