Copper catalyzed Ulmann coupling. It is ready when no more CO2 is generated from the reaction. It’s quite easily detected since, the question is, that is something bubbling in the gas bubbler (as seen on the gif) or no. If no, it’s done.

The blue color of the content of the flask is because of the copper salts used as a catalyst. At the end this pretty blue color fades and the whole reaction mixture turns into a red mass as seen on the following video:

For a bit more about Ulmann coupling: