I'm looking at a couple of litmus-test issues here:
1. June sales. Even a few NuSpidey supporters have noted that the recently reported April sales estimates seem a bit high, and they've speculated that this might owe something to Diamond using Battle for the Cowl as its estimate metric, rather than Batman itself. If this theory is true, June's reported estimates might give us a better idea of how the title is really doing (until then, though, I'm simply assuming that April's numbers are what they are).
2. Sales immediately following Amazing Spider-Man #600. Completists will be that much less likely to drop the book, the closer it gets to an anniversary issue, but once that mark passes, I can easily conceive of #600 being a perfect "jumping off" point for a lot of folks.