Unfortunately I don't have any pictures. I should take some next time I go back, at the end of this year. Might be worth re-evaluating. Having that said, I do recall several of the carvings well, and they had a distinctly different style compared to others in Tanum.
As for lichen growth, I couldn't differentiate the growth inside the carvings from that around them, so to 10-year-old me the carvings certainly appeared old.
But as an adult, I'd guesstimate that in that area, only ~100 years of lichen growth is enough for carvings to blend into the surrounding rock.
Edit: I am not sure if an lichen study was conducted.
As for lichen growth, I couldn't differentiate the growth inside the carvings from that around them, so to 10-year-old me the carvings certainly appeared old.
But as an adult, I'd guesstimate that in that area, only ~100 years of lichen growth is enough for carvings to blend into the surrounding rock.
Edit: I am not sure if an lichen study was conducted.