Justin Williams proposes a rule-of-thumb: don’t take more than 90 days on your 1.0.
Here’s the thing, though: I want, as a user, to see apps that take longer. More interesting, richer, harder-to-make apps have value and shouldn’t disappear from the world or be the sole province of corporations (who won’t make these apps anyway, for the most part).
So I’d modify the advice to say: don’t take more than 90 days, unless you can afford to and you truly believe your app warrants it. But be sure.