Jax's attacks grant a stack for 2.5 seconds that gives him 5-12.5% attack speed (max 40-100%; depending on level). Jax gains an additional 1.5% attack speed per stack at level 19.
No problem, I actually did the math to clear things up even more! At level 1, it’s 3.5% per stack for a total of 28% at 8 stacks. Level 4 is 5% per stack for 40% at 8 stacks. Level 7 is 6.5% per stack for 52% at 8 stacks. Level 10 is 8% per stack for 64% at 8 stacks. Level 13 is 9.5 per stack for 76% at 8 stacks. Finally, Level 16 is 11% per stack for 88% at 8 stacks.
Thanks for the heads-up.
http://www.mobafire.com/league-of-legends/forum/mobafire-support/wrong-data-406