In brief
transition-duration sets how long a transition takes. Its initial 0s means no change spread over time, even if another rule names the correct property.
Useful to know
transition-duration sets how long a transition takes. Its initial 0s means no change spread over time, even if another rule names the correct property. [1]
When an effect is absent, inspect the computed duration rather than only the selector. For multiple properties, read the duration list together with the property list.
Sources
For further detail and to check the information: