I finally found a time window in this frantic early spring to put the ugliest Elm in the world over a beautiful rock.
Question is, how many seasons should I wait to start lowering down the soil to expose it?

I would like to see that beautiful rock
Regarding your question - I would try every third of a growing season. Should have enough root growth to allow that I think.
But I never did it myself.
Might give it a go next spring myself.
Got 3 ugly elms waiting for their day.

Mimo wrote: I would like to see that beautiful rock

Me idiot me...
I haven't shot a picture of it before burying it!

Mimo wrote: I would try every third of a growing season. Should have enough root growth to allow that I think..

I guess you are right, these things grow like weeds in the warm weather. So hopefully the roots will be solidly attached to the rock in less than 2 years.
Thanks, Mimo!

No worries mate, we will wait