As has been said, I would carry on canvassing if someone told me there was already another window cleaner in the area.
I've picked up plenty of good customers who have had one but were unhappy with them.
Back on topic, when I started I canvassed my own estate first and worked outwards.
I started my current run 6 and a half years ago and haven't canvassed for five years now as most of the new work I pick up these days comes to me via recommendations and walkups.
John