Do we know when Small Gods is set, compared to the majority of Discworld novels?

Question

Small Gods portrays a time of great change in the Omnian religion, when the

Prophet Brutha comes to an arrangement with Om leading to a more peaceful form of the religion

This later Omnianism is clearly well-established by the time of Carpe Jugulum as evidenced by the Quite Reverend Mightily Oats; there are also references to a more violent past in some of the other novels featuring the witches.

But do we know just how long before the main timeline (most of) Small Gods is set? Do we even know for certain that the 100 years that passes at the end has finished before the majority of the novels are set?

Answer 1

In short - no. As hard as it is to prove a negative, and if anyone finds a definitive statement from Sir Pterry I will happily delete, but I can't find any primary statements.

Carpe Jugulum implies that the events of Prophet Brutha are historical, and one assumption is that the century mentioned at the end of Small Gods falls before the majority of the other novels.

However, there are characters referenced who appear in both Small Gods and Pyramids (some of the Ephebian philosophers), and Dr. Cruces is in both Pyramids and Men at Arms. Additionally, both Reaper Man and Small Gods state they occur in the Year of the Notional Serpent. The argument is given in more detail on the L-space timeline page.

It's possible to pick holes in either option (could there have been more than one Dr. Cruces?), but absent word of the author, it remains ambiguous.

As pointed out in the comments, Thief of Time explains that the history of the Discworld has been cut up, stretched, moved, and spliced back in by the history monks, giving rise to oddities of history where things don't seem to have happened when they should have done; this provides a neat in-universe explanation for any and all discrepancies.