Rofl cmon homie...I know people who never played Riven talk alot of shit about her on the forums but cmon homie...
Jax' E has a 16 sec cooldown at level 1 and he usually maxes it last. I used to max it sometimes vs Riven in previous seasons (maxing W is alot better though usually) but even then its still an 8 sec cooldown. In the normal case its a 16 sec cd until level 13, all of Riven's skill have far lower cooldowns and you can avoid it so easily by just stunning him after he activates it and then walk and E or Q away. I have played the Jax vs Riven matchup so often (at every elo until low Diamond) and while at some point Jax starts to autowin Riven should never get stunned by his E.
BTW his E doesn't go on cooldown until he activates it again to stun so in reality its a 17-18 second cooldown. (Unlike Riven's Q which has a 13 second cooldown but goes on cooldown after pressing it once).