If you create a custom item type instead of using Google's categories for item-types Google will not give an expiration date. So your item will not expire.
Google Base is supposed to show your Product Items (with links to your store) when someone search for that "product keyword" Its apart of Google's Universal Search Strategy. They usually show this near the top of the search listings so in theory your site might not rank on the first page but your products could. For example - you sell socks, someone types in "white socks" and your white sock products will show with pricing info etc in the first page of search. I been trying this out and had limited impressions on my items. Does anyone know how I can more impressions on my items in google base? Thanks!