A firm invites its top candidates; i.e.. the people they most want to hire, to callbacks first. They then look to the second pile and begin to schedule callbacks for mid-September, in mid-September if they think their class will not be filled, they begin to schedule more callbacks. So basically if you have not received a callback from a firm that has already scheduled several other callbacks but you have not gotten a ding then you are still in the running but you may be at the mercy of the quality of their other callbacks and how many acceptances they get. I know people that were still getting callback invitations and screeners into October last year--it can be a tricky game if you have an offer but would love to be at a different firm that has yet to ding or call you back. The 28 day rule should expedite the process a bit, but it should be obvious that a callback is good, a ding is not great, and hearing nothing is, well, nothing.