We can do 6 knots x 72 hours - that'll get us about 432 kM. Ideally, I'd prefer to have enough for 750 kM.
Check out your engine
rev's at 6-7 knots and look on the manufacturers data site to get the hourly fuel consumption
. If it's really critical, you might then want to test the actual fuel consumption
as the manufacturers data is theoretical.
I'd agree with comments above that you want at least 3 days worth at a nice cruising speed. Maybe go for more since you can fuel up in the US cheaper than in the Islands. Also, consider double tanks
& a good filtration system.