How long does the voyage from Hong Kong to San Francisco take by boat?

It all depends on the size of your vessel and it's fuel capacity, if it is not a sailing vessel. On a modern day ship it could take as little as 16 days to get there. On a small sailboat if could take 3-4 months or more. Again factors to take into account are: Size of your vessel, weather conditions, fuel capacity, cargo, experience, route and time of year.