After some digging, I found a further detailed timeline of Known Worlds history:
http://www.stront.f2s.com/fadi...age=177/article.html
Below are the specific year benchmarks pertaining to the Symbiot Wars. This information is mostly moot if we play during the Rise of Alexius, but I thought it best to set the record straight. Besides, it always pays to know your history. ;)
4900 The Symbiot Wars. Symbiots attack outlying worlds. Parasitic forces convert many planets, forcing the human populations to evacuate. The Regent moves quickly to defend Stigmata. The Imperial Fleet is dispatched and fierce battles begin.
4905 Doctrine of Exemption Exceptional and Doctrine of Temporal Exception. These documents allowed freemen and even serfs a greater (though still-limited) access to technology (mostly agricultural technologies and weapons for war against Symbiots and other aliens).
4908 Doctrine of Alien Rights. Aliens loyal to Humanity and the Church are allowed to operate with limited freedom. (This doctrine was passed mainly to increase the military recruitment of aliens to fight in the Symbiot War). The Church went too far with this in many people's eyes, and a conservative backlash quickly ensued, bringing the era of aggressive reform to an end (lasting from 4665 to 4908, only suffering a brief setback under Patriarch Halvor).
4948 Occultism is discovered to be a very efficient weapon against Symbiots.
4950s The Symbiot Wars come to a standstill, the Symbiots kept back at Stigmata. A purge of Symbiot groups and forces within the Known Worlds begins and is seemingly quite effective once Stigmata is "secured". Some call this the end of the Symbiot Wars.
4955 The Eskatonic Order is recognized by the Universal Church.
So there you have it. So much for my estimates! That'll teach me to trust a one-liner out of the corebook for dating the timeline of a war. If we do still end up playing during the Symbiot Wars, the game will begin in the year 4955, following the Church's recognition of the Eskatonic Order and dispatchment of the Manifest Light Legion to Stigmata - a year before the start of the Emperor Wars.