That's a great question. Thank you.
The short answer to your question is that there's no particular timeline, but with your indulgence, I have an interesting anecdote for you that provides a bit of colour to this.
During the last collection of data we photographed the city of Windsor, unfortunately, during their garbage strike. The city fathers were a little disappointed that the image they were presenting to the world was of a city with garbage piled up everywhere, and they actually wrote to Google and the correspondence was directed to me, specifically, saying, “Please, come back to Windsor. We want you to come back and photograph Windsor when there isn't garbage piled up everywhere.” So we in fact did do that. We came back to Windsor specifically to photograph it again.
I should also say that Toronto was partially photographed during their garbage strike. So if you go to Google Maps, you will see that Christie Pits Park had a whole bunch of garbage piled up in it. Toronto hasn't requested that we come back, but we did go back to Windsor because it was important to the people of Windsor that the Google cars return and a clean vision of the city be presented.