Advertising. The Service does not include advertising in the maps images. However, Google reserves the right to include advertising in the maps images provided to You through the Service, but will provide You with ninety (90) days notice prior to the commencement of advertising. Such notice may be provided on relevant Google websites, including but not
limited to the Google Maps API Blog and the Google Maps API Group (or such successor URLs that Google may designate from time to time). During that 90 day period, you may terminate your use of the Service, or provide notice of your refusal to accept advertising in accordance with Google's policies and procedures for providing such notice (which Google may make
available from time to time in its sole discretion).
1.6 Geocode Requests. There is a limit of 50,000 geocode requests per day per Maps API key. This translates to roughly one geocode request every 1.73 seconds. If you exceed this 24-hour limit, the Maps API geocoder may stop working for you temporarily. If you continue to abuse this limit, your access to the Maps API geocoder may be blocked permanently.
Source: Google - See http://www.google.com/apis/maps/terms.html