Having run Dwellers of the Forbidden City once - then it's once for me.
However, I have plans for an Aboleth in my upcoming campaign, so who knows? It might go up to twice.
The main problem with using aboleths is the same as any aquatic monster, really. You generally need to manipulate things quite heavily to get the players in a situation where they can meaningfully interact with them. An aboleth in a pond (as in I1) is pretty useless. However, as a mastermind behind a slaving operation rather than a one-off combat encounter, that I can see potential