As a base, enough so you are not trapped where there is a situation, and that depends on hazards, coastal, tidal, weather
For a single
handler - enough so if you accidently fall asleep that you will not end up in a situation where you are in a "situation". So, you calculate your boat speed * hourssleepdrifting and see what gives.
parallel to coast, i like to stay off 150 miles, especially if there is a current
to add to my gain. And stay out of everyones way (especially ships), so I can duck inside easily.
generally experts use their experience as "hedge words". newbies use absolutes. I like cookies and bananas...someone else likes coco puffs.
ANd dont forget the ZOMBIES!!!!