Never happen. They'll pander to the black activists (Holder already started that process) but there is simply no admissible evidence that Zimmerman targeted Martin because of his race. Or, at least not the sort of evidence that allows you to get a hate crime conviction.
The prosecutors in Florida already made fools of themselves by overreaching and over-charging Zimmerman due to political pressure. Holder and the DOJ are a lot of things but stupid and in need of further embarrassment isn't one of them.
There will be no federal charges. The investigation will continue for a good long while and in 8 or 10 months there will be a quiet press release/announcement on a late Friday afternoon stating that no charges will be filed.
Which is as it should be.