The ball will reach its maximum height when the ball's velocity is equal to 0 (slowed down due to the force of gravity). So, using some kinematics:
where a is the acceleration due to gravity in feet/sec^2
Solve for t and plug into your original equation.
Or if you know your mechanics and a bit of calculus, differentiate both sides and solve for t when (i.e. when velocity is equal to 0)