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.

The forumla

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