No one knows. I've never seen a study on percentage of hidden outdoor grows that get discovered. The plants need a lot of light so it can only be so wooded. They also need regular watering, unless you're talking about a rain forest. How likely are you to be seen going to check on the plant? If it becomes dry in the area, will it be suspicious if you're seen with a big watering can entering the woods? What are you going to do to prevent them from being chewed up by wildlife in the area? Will you be applying pesticides, and visiting regularly to make sure pests don't kill the plants?
More than once I've read about and seen grows get busted cause cops or someone, saw a guy going into a wooded area with gardening equipment. On the other hand, I've known of successful grows in wooded areas, but the forest was huge, human traffic non-existent and nowhere near a city.