You are correct that the maximum answer to that equation is 0.25. But i think you are incorrect in your assumptions about what that number is. That is an angle. Specifically 1/4th of a radian or about 14.32 degrees.
Look at this diagram:
The pendulum swings from point b to point d.
This gives us:
Just remember that these angles are in radians and you should convert them to degrees by multiply by
L = .5m = ab = ad.
So we need to find an such that ed=.01m (1cm)
I will assume you can take it from here once you figure out cd then solve for theta
Then plug theta back into the original equation to find t