For example, on a site that used to get around 2 million page views per day, the log size had 133,910,121 entries for only one week, and consumed over 38 gigabytes!
Source: http://2bits.com/drupal-performance/reducing-size-and-io-load-apaches-web-server-log-files.html
Other interesting links…
http://zoompf.com/blog/2009/11/performance-questions-to-ask-hosting-providers-log-file-access