Effect on length of day as the polar ice caps melt

The short answer is about less than a second.

You can do a quick-and-dirty calculation by assuming that the mass of the polar ice caps (roughly $10^{19}$ kg or about $\frac{1}{200,000}$ of earth's mass). We expect then that this would slow earth's rotation by that same amount (because the mass flows from the poles to the equator): $$ \frac{1}{200,000}\cdot86000\,{\rm s}\approx0.43\,{\rm s} $$

You can do a more complicated solution involving rotational kinetic energy, $K=\frac12I\omega^2$, but this will not change the answer significantly (increases it to something like 2/3 a second) than the quick and dirty solution.