Which chance does an item have, if the chance is not specified?

For most items the chance of an attribute is specified: Sky Splitter:

15–20% chance to Smite enemies for 600-750% weapon damage as Lightning when you hit them.

But for some items it is not specified: Thunderfury:

Chance on hit to blast your enemy with Lightning, dealing 279–372% weapon damage as Lightning and then jumping to additional nearby enemies. Each enemy hit has their attack speed and movement speed reduced by 30% for 3 seconds. Jumps up to 5 targets.

Which chance does an item have, if the chance is not specified?