So When did Allevrah Azrinae fall to the dark fate how many years ago did she become a drow and then go down to the city and take over House Azrinae?
AFAIK, they've left that vague. In my game, Kaer'ishiel told the group that General Allevrah left on an undercover mission to Garund 2 to 3 centuries ago. I assume that leaves her enough time to have risen to the head of House Azrinae in Zirnakaynin, considering how far the PCs are able to advance in the space of 3 months.