It appears you can set past versions of documents to expire after a certian number of days. I have not tested this. I researched it because I wondered if document histories were transient by default. (It appears that they are not - document histories are "indefinite" by default). As of Tavi 0.21 (and possibly before) there is a variable in lib/defaults.php named $ExpireLen. If you set it to an integer (say, 5), document histories older than 5 days will be deleted from the database. Of course, you don't want to edit lib/defaults.php itself, because it is just an example. You want to add $ExpireLen to your config.php file.

See ExpirationPeriod.

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Actually, documents expire in 14 days by default.