Roughly a 3-4 hour walk from boston at wander speed with swimming. The second picture is for comparaison purposes to see the light level differences. It goes down so slowly that you don't notice until your stats start dropping.
Stats are effectively lowered at a steady rate, food doesn't help heal the damage. Light such as torches or campfires raises the light level, but venturing further lowers the light level further. The healing of stats is also significantly lowered in speed, even with enough light to stabilize you.