Depends on the encyclopedia, but the general rule is each year. They are updated annually to include new events and findings. But, if you're researching something that is old - say the history of a president in the 1800s, it is unlikely that there are new facts that have been recently found and you can still gather good information. Science and technology advance daily, so encyclopedias become much more outdated in that regard.
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Depends on the encyclopedia, but the general rule is each year. They are updated annually to include new events and findings. But, if you're researching something that is old - say the history of a president in the 1800s, it is unlikely that there are new facts that have been recently found and you can still gather good information. Science and technology advance daily, so encyclopedias become much more outdated in that regard.