That may be a more complex question than you realize. Figuring what the boat can do and what the sailor can do often require different formulas.
There are basically two types of sailors: those who enjoy getting there, and those who enjoy being there. The first type often doesn't mind a slower passage
; if the winds lighten up and the knotlog only reads two knots, they are fine with that. The second type, who will inevitably be married to the first type, generally have a number etched in their brains below which they cannot tolerate dawdling. My wife, for example, begins to itch if the boat speed drops below five knots on a long trip, and if she's on watch there's a good chance that the engine
will come on once we drop below four knots. For her, the fun begins the moment we drop the hook and rig up the sun shade. And not a moment sooner.
It helps, if your boat partner is a "being there" type sailor, to have a big, fast boat and always run it downwind in the tradewinds. At that point, you can reliably pick a number, such as "150 nautical miles."
If someone tells you that they average anything more than 150 nm per day, you must consider one of five possibilities: (1) the person is wealthy enough to own too much waterline for their own good (I include owners of large catamarans in this group, btw); (2) the person pushes the boat and crew far too hard for their own good; (3) the person spends the greater part of his/her time sailing through gales; (4) the person is navigational challenged and really hasn't got a clue how far he sailed yesterday; (5) the person is a congenital liar.
I'm happy to report that there are only two congenital liars included among the membership
of this forum, and that they both claim to be "getting there" types, but we have reason to believe that their fuel
bills are higher than they admit.