It also depends on the height of the trees and your latitude. Can you get to this property to see for yourself? Luckily we're near the winter solstice now (close enough) so the sun is as low in the sky as it's ever going to get right now. If you can get to the property and spend some time there, you should be able to see how far back you need to clear trees from your site.
I'm guessing you'll be around 50 degrees North latitude. From a sun chart I'm looking at, for 48 degrees North the winter sun comes up at 8AM directly from the Southeast, hits 9 degrees off the horizon at 9:15, maxes out at 18 degrees at noon, drops back to 9 degrees at 2:45 and sets at 4 in the Southwest. Since you'll likely be a bit farther North, it's worse. To get 6 hours of sun there, you'd need to clear anything to the SE and SW that is higher than 8 degrees off of level. That is likely a long distance.
"Hundreds of years from now it will not matter what my bank account was, the sort of house I lived in or the type of car I drove... But the world may be different because I did something so bafflingly crazy that it becomes a tourist destination"
There's a great app I use for iOS devices called Sun Seeker. It's an "augmented reality" app which, if you haven't heard the term before, superimposes data on whatever the camera is pointing at. In this case the sun's path, like so:
Though I often have to calibrate the "compass" for the actual sun's location, it's an amazing visualization tool-- you can even change the date and time to see where the sun is at that particular instant. It may not be dead-on precise, but it has been more than good enough for me to use on a cramped suburban lot to understand what may occlude the sun at various times of the year in different part of my yard. I'm not in any way affiliated with the author; I'm just sharing a tool I find very useful!
This is a VERY unscientific observation, but one gleaned over many years, and that is the first 50 feet of our corn next to the forest is pretty well stunted due to the lack of light. I would say that it is 18 inches to a foot lower then the rest of the corn. Last year we planted our family garden in a new spot where ONE tree stood in the way of the westerly direction and we could not believe how stunted the growth was in the shadow, and we are only talking an hour or so per day.
But there are other factors too that you may not have considered such as the effects of PH on the soil the closer to the forest that you get. That can easily be compensated with extra lime (compost), but it does occur. You can also get some nitrogen fixing issues with the forest robbing your soil of nitrogen, but again that can be fixed with extra fertilizer (compost), but it does occur.