The Forgotten Realms have vivid descriptions of Ceremorphosis since at least Fiend Folio (2003) for most species, but The Illithiad (1998) before that shows Ceremorphosis of a Roper into a Urophion and deeper insights into the Illithid lifecycle. But that's not where the story began, just the oldest two mentions that are cited in the ceremorphosis article on the FR-Wiki.
The fact that Illithids are sexless and have no sex has been known since Into the Void (1991), while their reproduction via tadpoles was established in the Monstrous Manual (1993):
They are warm-blooded amphibians, and spend the first 10 years of life as tadpoles, swimming in the elder-brain pool until they either die (which most do) or grow into adult illithids. On an irregular basis, adult illithids feed brains to the tadpoles, which do not molest the elder-brain. Illithids are hermaphroditic; each can produce one tadpole twice in its life.1
However, Ceremorphosis, the turning of a humanoid into an Illithid wasn't established in either of those. In Fiend Folio, decades years after the inception of the Illithids, told us how the Illithid lifecyle actually works:
HOW ILLITHIDS MAKE MORE
The mind flayer life cycle is a closely guarded secret that few nonillithids have become privy to. Illithids begin their lives as tiny tadpoles in a briny pool. To reach maturity, the tadpole undergoes a transformative process called ceremorphosis. The process begins when a tadpole is inserted into the brain case of a host human (usually via the ear canal). The tadpole burrows into its victim’s brain, quickly consuming much of the gray matter and replacing the consumed brain with its own squalid tissue. In effect, the tadpole melds with the uneaten lower brain stem of the victim, wiping out all remnants of the personality and spirit of the host, while leaving the physical body alive and ready for habitation. Within days, additional morphological transformations complete the maturation process of an adult illithid.
Ceremorphosis completely replaces the original tissue of the victim with illithid tissue; when the transformation is complete, the original victim is dead, and no spell or power can reverse the process.2
The Illithiad, 5 years prior, did go into quite gruesome details for how the process looks and physical characteristics - in fact, it was a good part of this text that found its way into the Fiend Folio - but no illness stats or progressions. The best you get in this text is that it takes the tadpole three rounds to reach the brain after insertion and one hour after insertion you are dead, your body in coma and destined to stand up as an illithid in 7 days.3
However, as much as I dig, I can't seem to find an official depiction of the process of ceremorphosis as an illness or plague that PCs would need to resist, or as a parasite that could be cured, extracted, or otherwise removed. In fact, it seems that the Fiend Folio and The Illithiad deem ceromorphosis as rendering the character unplayable minutes after infection with a tadpole.
Is there an official rules text in any edition of D&D (but most preferably 3.5th) on how to handle the steps of Ceremorphosis?
1 - Doug Stewart: Monstrous Manual, Lake Geneva (1993), p. 251.
2 - Eric Cagle, et. al.: Fiend Folio. Renton (2003), pp. 90.
3 - Bruce R. Cordell: The Illithiad. Lake Geneva (1998), p. 11-12.