In the past, a few of my customers have had issues with dropping off google shopping.
There can be many different reasons for this. 404 errors - banned products - missing information - bad product feed - ect.
So if you are not checking on your feed on a weekly basis you may be dropped of the google shopping. If you are not able to see your site on google shopping then you should reinstate it with the link below after you fix the issues.
http://support.google.com/merchants/bin/answer.py?hl=en&answer=188482&topic=2475589&ctx=topic