References to descriptions of the formless realms in the suttas (or other Theravada texts)
...They enjoy it and like it and find it satisfying. If they’re set on that, committed to it, and meditate on it often without losing it, when they die they’re reborn in the company of the gods of the dimension of infinite space. The lifespan of the gods of infinite space is twenty thousand eons.


...If they’re set on that, committed to it, and meditate on it often without losing it, when they die they’re reborn in the company of the gods of the dimension of infinite consciousness. The lifespan of the gods of infinite consciousness is forty thousand eons.


...The lifespan of the gods of nothingness is sixty thousand eons. ~~ AN 3.116 ~~