How long can demigods live?
- Daedalus lived for thousands of years... by making a discovery for immortality, like Nicholas Flamel?
Circe lived for thousands of years... by using magic?Or is she a minor god?
- Anyone else?
They're probably not naturally immortal or Artemis offering immortality to Bianca is rather pointless. There's no real suggestion in the books, that I recall, that suggests demigods live longer than other mortals. (There is some suggestion that they tend to die young, especially those killed before they can reach camp.) The demigods in the Greek myths Riordan's books are based on also tended to live mortal lifespans. Hercules only becomes immortal after his mortal side is burned away, allowing him to rise to Olympus at his death.
ETA: Also Daedulus becomes 'immortal' by taking new bodies -- hence his disguise as Quintus, his fifth body.
There's VERY little canon information to go off of:
The average total lifespan seems to be very small due to majority of demigods (especially Greek from Camp Half-Blood) being killed off early in life:
“You—you’re a half-blood?” I didn’t mean to sound so surprised, but I’d never seen an old demigod before.
Quintus chuckled. “Some of us do survive into adulthood, you know. Not all of us are the subject of terrible prophecies.” (The Battle of the Labyrinth)
This is corroborated by Riordan's FAQ:
Q: If two demigods had a child, would that child be a quarter-blood, a demigod, or what?
A: Most half-bloods at Camp Half-Blood don’t live long enough to have children. Their lives are simply too dangerous. ...
The average natural lifespan (absent traumatic mortality) isn't discussed too much, but we have a couple of data points leading to believe it was same as regular mortals:
Daedalus seemed to have died naturally soon after his young nephew died an had a reasonably normal lifespan:
I thought about the old man I’d seen in my dream. He’d looked so frail, it was hard to believe he’d last another week, much less three thousand years (the dram was of right before his son died).
Then, he killed his young nephew, which means, he wasn't already old when Icarus died.
More directly, they mentioned several historical figures as demigods, all of whom had natural deaths and none lived longer than normal humans:
George Washington, Jack London, Harriet Tubman, Houdini, Frederic Bartholdi, William H. Seward, William Shakespeare, William Sherman.