Automatic duration of items onscreen

I have a 5-min video on slide. During that slide, I want an item that (appears/ onscreen 3 mins/ off / new item appears / onscreen 2 mins/ off). I can't change slide itself (because of the running video): so how do I time the duration of the items? (NB This is NOT an issue with animation itself, of the items and their entrance/exit, but the duration of the items onscreen.)