iii) How do we know that Sir Charles had stopped at the gate for sometime?

From the text:

“It was all very confused. Sir Charles had evidently stood there for five or ten minutes.”
“How do you know that?”
“Because the ash had twice dropped from his cigar.”
“Excellent! This is a colleague, Watson, after our own heart. But the marks?”
“He had left his own marks all over that small patch of gravel. I could discern no others.”