In runescape, what is the dragon full helm drop rate?

Ive been getting a lot of metal dragon tasks, and ive already gotten a visage. And i just got a mithril dragon task. I know that there isnt a specific drop rate for the visage or full helm and stuff like that. But if youve gotten 1 or 2 dragon full helms as a drop. Can you tell me around ow many kills it took for you?