Skill-Bonneti's Defense

I have a question about Bonneti's. It says that it ends when a skills is used..by this does it mean when you first activate the skill or when you finish casting the skill. By this i mean, if i had bonneti's on and i tried to cast something with a 3 second cast time without getting interrupted, would i lose bonnetis as soon as i cast it or after i cast it? thanks

You would lose it as soon as it is finished casting.

If you are interrupted during the use of that skill, Bonetti's Defense will end.

It ends as soon as you finish casting that skill..