It seems what was added to weston.ini https://cgit.freedesktop.org/wayland/weston/commit/?id=11ae2a3036bc95ff26b12a6d60f12514d8145143 was added right in the middle of idle-timeout now it looks like the default for idle-timeout is 300 seconds, and that it could be set on the command line with the -i option
Whoops, that's right!
Sent https://patchwork.freedesktop.org/patch/143753/
Merged as 65e57c93caf234235bd64d89ecae033526acddce.
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