Final answer:
The default value for the CSS3 transition-duration property is 0 seconds, indicating no transition effect will occur unless a specific duration is set.
Step-by-step explanation:
The default value for the CSS3 transition-duration property is 0 seconds. This means that if you do not explicitly set a duration for a CSS transition effect, it will not occur, as the duration indicates how long a transition should take to complete. If you want an element to transition over a period of time, you must set the transition-duration property to a value greater than 0, commonly specified in seconds (s) or milliseconds (ms).