Whilst the issue of data retention of search log data has been the subject of much discussion, notwithstanding the Art. 29 Working Party's opinion, that the Data Retention Directive 2006/24/EC does not apply to search engines, yet the retention policies of search engines continues to be a discussion point:
Google - 9 months retention policy
Yahoo - 3 months
Ixquick - 48 hours (as of 28/1/09 - no IP addresses are not stored)
Ixquick appears to be the preferred search engine for its retention policy, having been awarded the European Privacy Seal - whether other search engines will reduce their retention policy remains to be seen.