Very good question! It seems the symbiotic relationship and ability to get most nutrients from photosynthetic activity, water and nitrogen utilisation means the Lichen grows slowly. The complexity is related to the symbiotic interaction. What is turnover of the cells within the Lichen is tricky to answer also! Fungi usually cmplete their lifecycle in months rather then years once the food source is exhausted so the Lichen needs to grow at a rate that takes into account its nutrient sources- water, sunlight and particular trace ions.