With an hour's start, she gave Sanderson three-quarters of an hour to catch them.

Then, three quarters of an hour additional would be required for the run home--if they came back as swiftly as Sanderson had gone.
But she doubted that.

She would give them a full hour for the return trip.

That would make an hour and three quarters.
But it seemed to her that an age elapsed before the minute hand on the clock dragged itself one-quarter of the distance around the circle.
She looked out at Dale and his men.

The men were all standing, their backs to the house.