How long did it take the Ptolemaic, Seleucid, and Antigonid Kingdoms to coalesce and stabilize after Alexander's death?

Just a couple of decade, until 294 BC

Explanation:

Soon after the death of Alexander the Great, his generals started dividing his legacy. The strongest among them created their own dynasties.

The Kingdom of Macedon was created in 294 BC and with shorter interruption lasted until 168 BC. It wasn't a strong and rich empire, but it was unified and military advanced.

Ptolemy created his own Ptolemaic kingdom on the soil of Egypt in 305 BC and it lasted until 30 BC. It was the wealthiest among all Hellenistic states.

The Seleucid Kingdom covered the area of Persia and was military the strongest among all Hellenistic states.