How many times must the GeoCoordinateWatcher fire?

Question

My Wp7 (Mango) app needs to know the location of the user in order to search for the nearest object in the database. But I want to display the result on the welcome screen, so I need my location ASAP. I've written a code that starts the GeoCoordinateWatcher (on high precision and minimal threshold), and on the first firing it stops it, and sends the location to the webservice, which returns the object in question.

After I tested it, I went to present it, to the other side of the town. I started it and for everyone's amusement it showed a completely wrong result - I know why: it still thought I was at home!

How long does it take for the GPS to update? How will I know that I finally got my current location? How many times should I allow the GeoCoordinateWatcher to fire before stopping it?