I'll give him until Christmas. If we're in the bottom half of the table at that time, I'd expect him to be sacked as it'd be clear we'd made the wrong call.
If he finishes outside the top ten in his first season, I'd expect him to be sacked.
He's not a big name manager, therefore he has to prove himself, and if he doesn't he's out the door IMO. Everton should not be the safest job in world football just because we're a "nice club".
Patience, yes, but not stupidity. He's taking over a top six club. If he doesn't at the very least maintain that stature 18 months from now, he should be out the door.