4- the Germans continue walking but haven't spotted the Russians yet..
5- but at about 50m range they spot the first russian squad and begin firing..
6- The Russian view, they didn't return fire because of their very short arc and routed soon after this screenshot was taken, but in a proper game you'd remove or expand the arc as soon as you want them to start shooting..
Conclusion- In heavy forest on Hide, you won't be seen until the enemy closes to about 50m, but in thin forest you'll be seen at around 100m.
Of course for total concealment you could deploy your infantry deeper inside the forest if the tactical situation calls for it, but the the disadvantage is that they won't be able to see out into open ground.
Reminder-the tests were conducted on a bright sunny day, so we can assume spotting distances in all terrain would be much less at dusk/dawn/night and/or bad weather.