The "chat-poll-timeout" setting:

On an HTTP request to /chat-poll, if there is no new content available, the reply is delayed waiting for new content to arrive. (This is the "long poll" strategy of event delivery to the client.) This setting determines approximately how long /chat-poll will delay before giving up and returning an empty reply. The default value is about 7 minutes, which works well for Fossil behind the althttpd web server. Other server environments may choose a longer or shorter delay.

For maximum efficiency, it is best to choose the longest delay that does not cause timeouts in intermediate proxies or web server.