I think this should be 0s, that is the [initial value][1]. Also:

    Even if all units represent the same time for the value 0, 
    the unit may not be omitted in that case as it isn't a 
    <length>: 0 is invalid and does not represent 0s, 0ms.

https://developer.mozilla.org/en-US/docs/Web/CSS/time

[1]: http://www.w3.org/TR/css3-transitions/#transition-duration-property