OBS does not have functionality built in for this sort of delay.
Most who do this kind of thing embed the .webm into a local webpage, and use CSS to control the video playback and delay between loops, IIRC.
Another workaround is to make a one minute long, blank video, and add it to the media source playlist five times. Then just leave the whole thing on loop. It creates more overhead (as it is always playing a video, even if the video contains nothing), but it's simpler for those who don't know how to do the webpage version.
ANOTHER workaround is to add the .webm as a media source, then set up a toggle hotkey for its visibility. Then set up an AutoHotkey script to push that hotkey every 5 minutes (and set the video up as 'restart playback when made visible').