A very good question. I admit for a long time, it was a hunch fuelled by the physical transition of Khrimm and Ascilia from pre-puberty children into fully matured adults, but I think I finally found my proof in the Level 20 Goldsmith quest, specifically some of the NPC banter.
Additional Context:Count yourself lucky that you were not here some ten years ago, to suffer the strife which engulfed Eshtaime's Aesthetics.
In our darkest hour, there was a very real danger that the Order of Nald'thal would seize our name and assets.
- I would conclude that the involvement of the Order of Nald'thal would be in response to the loss of Warburton's corpse, which they had acquired at great expense, and a desire for compensation.
- Throughout the goldsmith story arc, you learn of how Nielle's father became deposed as guildmaster following a fallout with Lord Lolorito.
- Given that Lolorito guards Heartstrike like a man possessed, its theft and subsequent damage would be one of the chief things that could truly frustrate the owner of Esthaimes.
- In the German text, it speaks of a "good ten years ago". So we're talking on the more lengthy end of ten years rather than the shorter end.
- Now, given that in the Ul'dah story arc Niellefresne is at the peak of his prominence and popularity, we must assume that it takes place before this event that was the likely trigger for the deposition of Nielle's father as guildmaster.
Getting back on to the other side of the argument, if we assume that the events of the Ul'dah story arc occurred after the fall of the keeper (ten years ago), allowing Ascilia to have borne witness to it together with Warburton and bear the message back to his Order, and if we agree that the events unfolded before Esthaime's darkest hour (ten years ago) in order to facilitate Nielle's level of personal influence, then the Ul'dah story arc must have occured no more or less than ten years ago - 1562.