That would depend upon a lot of factors. Your visibility is going to be impacted by things that block line of sight, weather, height, etc...
Generally, you can see about 3 miles of flat earth around you. When things stick up out of the earth and are large, you can see them from a furthe distance. If you fly up in the air (or climb something tall), you can see farther. From 100 feet you could see 12 miles, and from 500 feet up you could see 30 miles.
If all you're trying to do is decide whether to label something as jungle, plains, water, hill, mountain or desert, you might be able to get some elevation and place those labels for many hexes around you.
However, if you want to know if there are ruins in the jungle, a village amongst the trees, etc... you likely need to be much closer.
Here are some of my basic rules for Hexploration:
1.) I use 6 mile hexes, but each hex is subdivided into 7 zones. C (center) and 1 through 6 (in clockwise order). Each subhex is roughly 2 miles across. I use these subhex notations to identify which part of a 6 mile hex things are located within when hexplored. I think of the map in 6 mile hexes, but these subhexes are used to organize the finer details, if that makes sense.
View attachment 146242
2.) There are four tiers of 'hex information' - Unexplored, Seen, Explored and Searched. An Unexplored hex has never been seen and it could be anything. A Seen hex has been seen from a distance, but the smaller details of the hex are unknown. You could say it is forest or that there is a large lake within it, but not know if there are ruins in the forest or a village along the lake. Explored has been seen to a level where landmarks that are not concealed are now known, and Searched is a level where you have a good chance to uncover hidden secrets like an overgrown mine shaft, a faerie ring in a dense forest, or ruins at the bottom of a lake.
3.) If the PCs move at typical speed, have access to no flight, and are dealing with easily traveled terrain, it takes 2 hours to See a hex that is relatively clear. It takes a day of adventuring to Explore a hex. It take three days to fully Search a hex. If the terrain is rough, we double that time.
4.) If they have access to elevated sight, they can see much further, potentially Seeing many hexes. From 50 feet they can see hexes around their current hex. From 125 feet they can see 2 hexes in every direction. From 250 feet 3 hexes. From 400 feet 4 hexes. From 1000 feet they can see 5 hexes. From 1500 feet 6 hexes. This often is achieved with the assistance of flying familiars, a flying PC, etc...
5.) Mountains can be seen from 8 hexes away if there is line of sight (no jungle blocking, etc...)
6.) For each thing that is available to be discovered on a map, I indicate four things: Location (Hex/Subhex), Chance detected when seen, chance detected when explored and chance detected when searched. Some thing will automatically be found when explored or searched, while there is a chance to detect it when seen. Other things have no chance to be noted when seen, a slight chance when explored, and a greater chance when searched.
7.) The chance to detect is labeled as a percentage chance, and when I roll to determine whether it will be discovered I add either 1.) The best passive perception in the party, or 2.) The result of a perception roll (which is likely at advantage due to assistance) from the party. The only ones that can participate are the ones that are looking. If the detection requires manipulation (as in the ruins are beneath something that can't be seen unless an obstruction is removed) I substitute investigation for perception.
8.) The party can split into sub groups and explore in smaller groups - but splitting the party can be dangerous. If your 6 PC party wants to explore 6 hexes in a day, they can do so ... but they may not all survive.
How does this play out in "real life"?
Usually, one PC has a way to see from a high elevation (raven or owl familiar, aarakocra, flying tiefling), so there are a lot of "seen" hexes right off the bat. This influences where they want to explore and search. My DM map of the area tracks what has been explored and seen, and I give them a separate map (consider using these:
Tabletop Games | Headless Hydra Press) to show what they've seen. My map has checkboxes in each hex for seen, explored and searched.
PCs often miss things in hexes, even when searched, because they just fail the rolls. They have other ways to uncover these secrets via speaking to native creatures/beings of the area, so I let that occur naturally.
60% of my hexes are "interesting" as a general rule - enough that there is stuff to do, but not so much that PCs feel like they likely missed something if they do not find anything.
