I'm going to suggest a date: June 12th. They still haven't faced the Phillies, or the Mets at Shea. (They are 4-4 vs. the Mets and Braves, while beating up on the hapless Nats--which is exactly what you expect a good team to do.) After the series in Cincy, they come home to face the Royals, DBacks and Giants, followed by a 9 game roadtrip against the Phillies, Mets and Braves, starting May 26. Then they return home to face the Reds and the Phillies. The last Phillies game is on June 12.
Right now, the stats imply that their record is a temporary illusion--their defense in particular is suspect. They give up far fewer unearned runs than expected, based on the number of errors they've committed. And its hard to put much faith in the starting pitching--they've been good for the last couple of weeks, but can they sustain it?
This has been an unexpectedly hopeful season for the Fish. Even if they fade, as most people expect, there has been a lot of positive stuff: the Hanley contract; Olsen's turnaround; Beinfest's ability to grab marginal players like Cantu and Hendrickson and get production out of them; Miller's improvement; the bullpen, especially Pinto; the improved (to almost league average in terms of unearned runs), and perhaps still improving defense (even after Cantu's error last nite).
At least for now, the Cabrera/Willis trade looks pretty good.