IP addresses of anonymous editors are recorded in the database forever; this is problematic for both legal and moral reasons, given that IP addresses are often personally identifiable information. In previous wikitech-l conversations about hiding IP addresses, it has been argued that it would make the job of recent changes patrollers and other vandal fighters unacceptably difficult. That kind of utility quickly decreases over time, though; the IP addresses of months-old edits are rarely useful.
A MediaWiki operator will probably store all kinds of personally identifiable information (web server logs etc), but will discard them after a certain period (90 days is typical). Similarly, MediaWiki could discard IPs (replace them with random tokens) or otherwise obfuscate them (e.g. discard last byte) after a certain timespan.