When you want to show motion, one fifteenth of a second is the kind of time you need to think about.
Of course this depends on:
- focal length of the lens
- how fast the subject is moving
- how close you are
- how steady you are
..but in general, 1/15th is a good time to use.
To show movement, rather than to freeze it. Like in this snap of the London Heathrow Express:
Not showing movement (shooting at a fast shutter speed) would show a “stationary” train – which here would be a big mistake.