To quantify:
There is a base chance of 0.1% for forest to spread, per square, per turn. This is cumulative based on the number of adjacent forests, so a square in the middle of a 4 forest cross has a 0.4% chance. If there is a preserve in one direction, the chance increases to 0.8% per square per turn. (or 3.2% if in the middle of a 4 preserve cross) This means, assuming only one adjacent forest with preserve, it will still take on average over 100 turns for the forest to spread to any given square.
Finally, all these probabilities are cut in half if there is a road.