For example in Wikipedia it mentions: "The inhabitants of these realms are possessed entirely of mind". Also, "They do not interact with the rest of the universe".
In "In the Buddha's Words" (Bhikkhu Bodhi) it mentions "The lifespans ascribed to these realms are respectively 20,000; 40,000; 60,000; and 84,000 great eons".
I'm looking for that kind of descriptions of the formless realms but in the suttas (or other source Theravada texts).