Timer on Trynd and Renekton passive (and maybe Gnar)

So, Rengar has a timer on his passive that shows when he will lose his stacks of fury. I feel like that would be a good addition to other fury users so they don't have to guess when they need to attack to keep fury going, or how long they can wait between autos before it resets.

Edit: Just a clarification, a timer on the passive icon on the HUD.