I'd say about 10-15 minutes.
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I've still got some posts an hour old showing as "new"... I think it varies
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If you stop using the forums for a certain period of time (which I think is about 10-15b minutes) all the posts older than the time of your last visit will become old. Alternatively, you can press the "Mark all posts as read" link on the index page.
Logging out will have the same effect, but it will be instantaneous.
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