Whether its Koschek's orbital or Faber's hand or Sylvia's forearm, when is it reasonable to quit rather than risk a permanent injury.
Yes, 100K is enough to live on if a fighter can't fight again.
You can quit at any time.
Just don't ever, under any circumstances, back down.
If it's quit or take permanent, career/life threatening damage, quit. 
If you do it for fun then you quit when your hearts not in it anymore. If you do it for money you quit when you made enough. If you do it for fame then quit after you get your ego broken. If you suffer an injury that stops you from fighting just start coaching. It depends on the person.
Depends if you fight for the money or not.
When you're not 2 Legit.
It is never ok to quit...
...but everyone should know when it's time to stop.