What is the time dilation ratio that Noriko and Kazumi experience?
The center of the galaxy is about 8,000 parsecs away from Earth. The Gunbuster was heavily damaged while completing its mission, but was able to escape the black hole. However, they had to have spent at least 33 days at sub-light speed trying to outrun the blast while also trying to long distance warp back to Earth after finding suitable ethereal space for warp.

So 12,000 Earth years have passed. For Noriko and Kazumi only 33 days and a few hours had passed.