There's no timeframe reference except that Anders talks as if the memory is fresh.
His band likely formed after his conversion and probably are from around Elturgard, which isn't surprising given Zariel's connection to the city of Elturel.
As a paladin of Tyr midway between Elturel and Baldur's Gate, he was most likely an Elturgard Companion Knight and converted either before the city's descent or, holding a grudge, converted in Avernus itself, maybe even directly in Zariel's presence. If the latter, he's been fallen mere days or weeks.
And during Elturel's descent could have been where and when his band of mortal Zalielites was formed.
Devils have extreme difficulty plane-shifting to mortal realm, so the group probably came back within the city of Elturel when heroes saved it. Zariel has some foresight (in the adventure there is a possibility of her giving the city up in exchange for a better bargain), so maybe she instructed them to enter the city to return with it as her mortal agents?
This is really excellent detective work. It never occurred to me that we were standing on the doorstep the chain of events that led to their fall. I am going to consider that actual unless Larian says otherwise.
My working hypothesis is that the fall is even more recent and was precipitated by Zariel seeking Karlach and offering her aid to travellers at the toll house (including Anders) against the gnolls specifically to get their help to find her. That is, finding and killing Karlach was the price of Zariel’s aid. But I’m also going to pay more attention the next time I play to check whether there’s anything that either confirms or disproves this. I guess one thing that would disprove it would be if Zariel would have no way of reaching out to make such a deal, but I don’t know what means she might have for cross-planar communication.
"You may call it 'nonsense' if you like, but I've heard nonsense, compared with which that would be as sensible as a dictionary!"
