I don't think Spock "arrived." He evolved, and at a certain point during TOS he stopped evolving in a very interesting fashion - mainly when Kirk's dominance on the show was rather forcibly reasserted. Spock became the sidekick with a few quirky mannerisms.
A better question is when the writing for him stopped being interesting - I'd say after "Amok Time," with an honorable mention for "Journey To Babel." He's used effectively in some episodes after that, but the "inner life" that Nimoy has been talking about so much lately in connection with the current Trek movie just wasn't in sharp dramatic focus in episodes like, say, "The Tholian Web." He has a few good scenes there, as one example, but they don't really go far beneath the character's surface.
A better question is when the writing for him stopped being interesting - I'd say after "Amok Time," with an honorable mention for "Journey To Babel." He's used effectively in some episodes after that, but the "inner life" that Nimoy has been talking about so much lately in connection with the current Trek movie just wasn't in sharp dramatic focus in episodes like, say, "The Tholian Web." He has a few good scenes there, as one example, but they don't really go far beneath the character's surface.