Each share includes an expiration date chosen at share creation time. This is an absolute (as opposed to relative) timestamp which indicates the time after which the hosting server may delete the share with no ill consequences.
Expiration dates should be chosen based on how long the introducing server wants the data to last, considering the file size and likelihood of finding a server willing to make the trade.
By allowing the publisher of the document to set its expiration time, Free Haven distinguishes itself from many of the related works such as Freenet. We maintain an entirely content-neutral approach to the data stored in the system. That is, in the implicit contract between servers, each server agrees to store data for the other servers without regard for the legal or moral issues for that data in any given jurisdiction.
Designing a protocol which encourages content-neutrality may well mean that server administrators are less liable for the content on the network. Examples include common carrier law, wherein phone companies are not responsible for the content they carry over their systems. However, our strongest argument for maintaining a content-neutral system is that we think this is the most useful approach to a persistent anonymous data storage service. The Free Haven system is designed to provide privacy for its users; rather than being a persistent publication system aimed at convenience, is it designed to be a private low-profile storage system.