In my How to Create a Fantasy Sandbox I gave an estimate of twenty hours to complete the list. After reading Flynn's excellent post on creating a sandbox I realized that I should talk more about that time.
That time is based on the size of the region you choose to detail in step 4. In this case roughly 200 miles by 150 miles. The work involved increases or decreases geometrically the larger or smaller you go. This because the amount of stuff you can detail increases by how much AREA the map covers. So doubling the size of the map means four times the detail. Conversely halving the map means a quarter of the work. Something I learned when doing Acheron (a half sized map) vs the other three maps in the first Points of Lights. And is why for the blog, I just focus on a single small island for the detailed examples.
For the person trying this for the first time I strongly recommend starting small. Detail a 50 by 50 mile region centered around a barony or a city-state or an large island. Instead of the three dozen or so locales that will come out of my initial recommendation you will have only a dozen or so. The same for the detailed town, start out with a smaller size. Another option is to opt for the larger size and just have a sparse sprinkling of locales.
In his post, Flynn mentioned Zak S minimalist sandbox approach.
Create a map with interesting names
Create a couple of interesting encounter charts for different regions.
That can work if you feel you have a good Bag of Stuff in your your head. However most people need a little more structure than Zak's list. The important thing to remember about my "How to Create to Sandbox" steps is what they cover. I organized it the way I did because eventually you will need to come up with everything on that list in the course of a sandbox campaign.
The questions you will need to answer is that are these issues something you want to deal with now or later. Before the session or on the fly as needed? A detailed writeup, a sentence or two, a random table, or wait to make it up out of your head during the session.
I can't answer these definitively because the exact mix depends on you, what you know, and what you are good at. All I can tell you is to think about the issues before the campaign so you are not overwhelmed during the session. And of course give some tips on what I did in hopes that it will help your own game.