To elaborate a bit, Steve is referring to the links returned by createTemporaryDirectUrl, which always expire after four hours.
The links returned by createShareableUrl do not expire by default. Users can set expirations on these links manually, using the Dropbox web site, but this isn't currently available via the API. We'll consider this a feature request for that though.
Re: How to setup expire of shareable url for dropbox resource?
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