Christian,
May be service plug-in interested in timing interval should be the
ones responsible to enforce time-out. They would get all requests
matching the rule but only act when the timer has expired.
In this case, you would end up with forwarding all messages to the
service plug-in, although the service might modify the content only
every <n> minutes (per user). This is a disadvantage especially in
case of a callout service. Imagine you want to add/modift/remove
content only every 30 minutes (per user), so it would be unnecessary
to forward all messages from this user in-between these 30 minutes
intervalls.
-Markus