Could be anything between the Darkin War and now, which of course is a lot of time, but hey there's ways to narrow that down. Considering there were excavators at the tomb, this place must've been buried for a while, yet the darkin lass had escaped, indicating it'd been enough time for the tomb to be reconcealed by the desert between her escape and Varus's arrival. Considering it's a desert sand would likely bury it faster than earth would, so providing it was located in a part of Shurima that is particularly sandy (which is likely), then the process might've not taken much time at all, maybe a few weeks/months.
Considering the tomb is near one of the rivers flowing from the capital, it stands to reason that potentially, Azir's reestablishing of the waterways led to the tomb originally being uncovered due to the geological changes inherent to an ancient, dried up river bed all of a sudden having water flow through it again, putting her as being freed fairly recently.
Beyond that if we assume the Darkin War occured before the Rune Wars, than her tomb might've been revealed back then, perhaps by accident, perhaps intentionally.
I'd put it more to the whole Azir ressurecting Shurima though cuz the river proximity makes it seem more logical than hedging a bet on some random Rune War occurence leading to her freedom.