Engulfing the past

Back on our Leeds Liverpool Canal quest. Today we walked from Cherry Tree Station in Blackburn to Chorley. Bridges 95 to 76 to be precise. (It was a lot more than 19 as they've built more since the numbering started. We even saw one with 'aaa' after it today).

This milestone has been engulfed by a tree. How long before we can no longer see that it is 47 miles to Liverpool?