If you have 50% water, you can reduce that to about 9 years.
Consider the Skyrim example brought forward by Chosker above. While slightly larger than what you’re suggesting, it’s roughly the same. It took a team of about 100 people more than three years to complete. So, that’s 300 years if you work alone.
The way around it of course is to have a world that’s largely empty. No particular points of interest, nothing of significance to do, lots of duplication et cetera. Then you can complete it in a few years, and end up with something really bland and boring.
In other words, if you want a big open world that’s actually worth playing, you need a big team.