as a ruleI made this up today wrote:
There must be no active sprites or commands later than when a storyboard or video appears to end. Black screens or still backgrounds lasting for longer can feel endless to a player.
there are currently no rules regarding the end time of storyboards, making things like this: https://osu.ppy.sh/beatmapsets/796338/#osu/1672217
technically rankable even though a player will be just stuck looking at a black screen for 46 seconds due to the storyboard not properly disabling the particles when they were no longer in use and were offscreen (they're set to F,0,126739,,0 which is much much later than when the map and storyboard visually end which is at 79980 ms )
i'm not sure if this wording is the best it can be but yeaaahh i can't think of anything better atm
the same issue happened on https://osu.ppy.sh/s/792085
and a similar issue happened on https://osu.ppy.sh/s/591442 where there were commands lasting for like.. 10 hours after the end of the map, but in that case, since they were faded out, the storyboard did end in play properly, but it apparently caused other issues in relation to loading the storyboard, lag, etc...
a background lasting for a few seconds or so would be ok, but I'm not sure how to define just /how long/ is ok or in which cases it'd be acceptable.