How Does Focus Gain on Weapon Work?

Wicked Feedback
1

How Does Focus Gain on Weapon Work?

I have a sword that say: Gain 85 Focus.
How many swings I have to do to get that focus?

2

I guess each hit you’ll gain 85 focus?

3

Well thats what I thought but it doesnt seem like that cause I have runes that cost 50 focus which would mean that after every hit I get to use 1 rune.
But its not like that so I have no clue.