Do we know WHEN the polarity of the earth will change again?

The last reversal happened 780,000 years ago, and many scientists believe another has already begun.  They generally take just a few thousand years to completely reverse, but we will not have to worry about it too much for another 1000 - 2000 years.

No, this doesn't have anything to do with the gradual temp increase because the Earth's magnetic field is due to currents in the Earth's outer core.  The outer core's temperature isn't changing any noticable amount.

it should have already happned. it usually happens once every 250,000-500,000 years. but this last one has been delayed and is expected to happen within the next now -1000 years. when it happens it will happen fast. the lopsided-nes of the rotation of the earth on its axis and the cooling of the poles causes more weight at the poles and the earth is getting out of synch. if you take a pool ball and glue a heavy coin on the top then get it spinning wicked fast, you have a small semi accurate model of the earth. if you spin it it will eventually fall over so the coin will be on the outside of the spin. thats what will happen and it will severly affect the core of the earth and tectonic plate movement. and may cause massive eruptions from thousands of volcanoes and huge super earth-quakes that i dont wan to be around for.

We can't predict polarity reversals just like we can't predict earthquakes, the Science of Geology just isn't that advanced. When it does happen it generally takes several tens of thousands of years to complete, and in the meanwhile, minor dipoles replace the major (polar) dipoles. It has been a long time since the last reversal and there is some evidence that the polar dipoles are losing strength, thus a reversal is probably due, but we won't be around to see it completed.