Yes if she has max CDR at lvl 18. at lvl 1 the shield would window would be 8.5 seconds. She also has to be ahead for that shield to mean much since she has to have damage for it to scale, since you dont generally max e first.
She's not overpowerd, currently she's in the best stat she's ever been in terms of balance, since her strengths and weakness's are really well definded, not to strong not to weak.
I assume you just think she's over powered because you got buttfucked top lane against her, fed her, and the riven proceeded to destroy your team like every other champion thats fed does.