Hello all!
You gain progress toward this achievement as your ship sails across the map when you have an active Order of Souls voyage running. You can track progress by looking at the in game commendations for the Order of Souls faction in your reputation page.
For context, sailing top to bottom the entire map is roughly 5 nautical miles.
Obviously, the more fun way to get through with a chunk of this achievement done is to activate a souls voyage while working towards the tall tale achievements. Both quests can be active on the table at the same time. Another option is to pursue the following achievement and after you get it, you can see how much left you need to grind after that point.
For those looking to semi-idle this one, I have some numbers:
I sailed my sloop in circles (so wind isn't really a factor) and recorded that it takes roughly 2 min and 29 seconds between "nautical mile +1" notifications to appear at the top of the screen. If this is multiplied by 1000, this will take around 41 hours and 24 minutes. Update - The spinning in circles method doesn’t seem to work anymore. I would recommend finding a North to South or East to West path in Safer Seas and just sailing. If you are trying to idle this a bit, time-out is 10 minutes so rubber band your controller or something to spin.