Provided that the rest of the "Establishment" candidates are also polling single-digits, and that Bush isn't faring particularly worse than the rest of them, he has no reason to drop out. The "Establishment" certainly isn't going to desert him for Trump, and it's almost just as improbable for them to turn to Carson, Cruz, or Huckabee until they were left with no other choice (i.e., at the end of the primary season).
I don't see Jeb going anywhere, then, until either another palatably reasonable candidate opens a significant lead over their peers-- not necessarily all candidates-- in the polls. There simply is no reason for him to do so, because unless that were to happen, his constituency would have no reason to drop him.
That's because Bush's presence is stealing oxygen and money from better establishment candidates like Marco and even Kasich. If those two got half the legup as Bush, they'd at least be competitive for second place. Maybe.
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