From here on, it's simple proportion. Since J = Iω, and the earth's angular momentum stays constant as long as there is no outside disturbance, any increase in I will be offset by the exact same decrease in ω. The change in I, 6.22 E+32 kg m2, compared to the total moment of inertia of the earth, 8.0095 E+37 kg m2, amounts to an increase of 7.77 x 10-6. So to conserve angular momentum, we need to slow the earth down by the same amount. There are 60 x 60 x 24 = 86,400 seconds in a day, so a decrease of 7.77 x 10-6 amounts to 0.67 seconds or about 2/3 of a second. The day will become about 2/3 of a second longer. We'll have more time to get stuff done. Like pile sandbags along the coasts. Yay. The actual figure will probably be less because ice below sea level doesn't contribute any effect - it just melts and is replaced by sea water.