There were some stats from RG a couple of years ago that said Novak averaged around 3ft (or roughly 1m for those metrically minded) on a rally ball. Not that it really tells you anything, as we have no idea what his intent was in general or on any specific pattern.
I tweet - @ashtennis guru (no spaces)
I Shoot - www.flickr.com/photos/ashtennis guru/ (again no spaces! grrr)