...and then you get distracted by George, don't do that*, and it becomes an elastic four minutes, thirty seconds....unless you deploy a handy little online countdown tool like E.gg Timer.
Or you could get kids to open a new window on their machine and set individual timers. Although that could have a cacophony effect...
* as referenced here. Including a link to Joyce Grenfell's original monologue, lovely (and all-too-accurate) observational comedy.