It took me approximately 800 in-game days to explore the entire map. That includes time spent building bases as well, so you could probably do it in less time if you were truly dedicated to the task.

That being said, I'd recommend against doing this as it will affect where things can spawn on your server as new content is being introduced. I've been told that mountain caves won't appear in already-explored mountains and other similar assets (this doesn't included monster spawns) that are added later will also not show up in these areas that have already been revealed.

Basically, I screwed myself and my server by exploring the entire thing. Maybe this will change in the future, but that's just a hope I have for now. There's mods that're supposed to fix this, but I haven't been able to verify that they work.

Thanks for coming to my TED talk.