You generate 1 stack every 90 seconds to a maximum of 3 stacks after four and a half minutes.
Ranged champions can only have a maximum of 2 stacks after 3 minutes.
Using the active removes 1 stack.
I don't think that's a great idea, mercurial scimitar is actually used sometimes during competetive play and it can be an extremely effective item even as it is now. Giving ADCs 2 instantaneous cleanses in one item seems ridiculously strong. I don't think the issue with Quicksilver/Mercurial is that they are weak items, I think they're just very hard to use.
this dosn't sound horrible untill you imagine a 
or a 
just ignore all your cc
