At a cost of 25 gp every 8 hours. Let's say 16 hours if extended.
Not that his
nondetection does anything unless you target
him with divination magic.
His lair, teleportation circles, friends, family etc etc are all fair game, as are his Magen minions and the Celestials he seems to have working for him, plus also all of his Simulacra (who definitely know his name, daily routine, where he lives etc).
I could simply
Plane shift one of his minions, relatives, friends or allies in to Celestia, and then compel them to tell me everything they know about him
(in a
zone of truth or via
geas or similar) or simply use divination magic such as
Scrying to scry on the minions.
Notably,
Geas and
Zone of Truth are not divination effects (not that the minions have nondetection anyway) so non-detection wont help them.
The Celestials magical compulsion to serve him would be broken in my AMF, and sharing my alignment, they'd likely just tell me what I wanted to know anyway.
I could also simply enter the ethereal plane myself (via
plane shift) and travel to wherever he is, and then
plane shift out to the material plane there and whup his ass as well, but I guess some of his Celestials he seems to have voluntarily working for him have Truesight (his magen and Simulacrum dont though) so they might see me coming.
But as long as he's happy to hide under a rock playing solitare and doing nothing else other than repeatedly casting
nondetection for the rest of his life, that's probably a fate worse than the aforementioned beatdown with the rubber phallus in Celestia.