Some crustose lichens, such as map lichen, grow that slowly Peter. There are many others that grow far more quickly... up to an inch a year, like the ones I can see, on a 20yo palm tree, about 20ft from my desk.
Lichen grows about an inch a century - perhaps we should set up a lichen cam?
There's some lichen on a pavement near me which has been there as long as I can remember - it's supposed to be a very good indicator of air quality / pollution levels