lichen growth rates?

[Joseph Howell]

What is the general growth rate of lichens? Say for example, how long does it 
take for a lichen to cover the exposed surface of a rock? Or is this question 
moot, due to different growth rates between species? I'm asking mainly because 
archaeologists (of which I am one) often use the presence of a lichen-covered 
cobble as an indicator of a sites general age. But I've never actually had 
anyone claim to know just how fast lichens grow, so I've always been a bit 
suspicious of this - it sounds more like "outdoor lore". 

Any info? Thanks.