Here is some info I found when researched how fresh SE results are.
Google seems to be performing the best here. If site has any PageRank most pages will be indexed every day. Not so good for deep pages and websites that don’t have good links pointing to them. Such pages might take up to a month for Google to reindex.
Resolution: live results. You will not be able to find anything outdated in the top of results list
MSN/Live seems to go through every website anywhere in 3-10 days. How it is choosing frequency is unknown yet and not so obvious as for Google.
Resolution: almost live results.
Yahoo index update frequency is unknown. It’s always random, sometimes it stops coming to website for a month.
Resolution: sometimes live results. Sometimes you can find some old pages that are already out of index in all other search engines.