More details about expiration

[rfc2822]

• How long by default
• Can the client request an expiration? → Probably yes, makes sense because different clients have different profiles. A sync client like DAVx5 wants long-term subscriptions, but a Web client can be closed and maybe doesn't need subscriptions when in the background, so it may request only a short time and refresh if needed
• Can the server restrict expiration? → Yes, otherwise clients may request practically unlimited subscriptions
• Does the actual server-chosen expiration date have to be explicit? → Yes, because otherwise clients can't rely on it and don't know until when to re-subscribe.
• Method to query the expiration date → Probably return the subscription details (including expiration) after creating/updating

However servers should at least allow some reasonable limits which should be mentioned in the document.