This is a great litmus-test type question, because narrow-minded ideologues (Republican or Democrat) would never be able to answer it.
Really, as long as he doesn't screw up the country as much as Bush, he will have succeeded in my book. This means, 1) fund science, 2) improve our international image, 3) get us out of endless wars, 4) improve the economy.
If he stops or fails to do any one of those things, I'd probably stop liking him. I'll give him some leeway to accommodate for obstructionist Republicans who'd rather see the US in flames then Obama succeed at anything.