He wasn't arrested in the first place, because the police were unable to articulate a reason, based on evidence, to doubt Zimmerman's account.
If he had been arrested, "speedy trial" provisions of law kick in, and charges have to be brought in days.
I don't know the statute of limitations on manslaughter, but a suspect can be free for at least a year (indefinitely for murder and tax offenses) without being charged.
The delay in this case isn't really a delay yet. The case was scheduled to be put to a grand jury on April 10, by the original state (prosecuting) attorney, Wolfinger. The governor replaced Wolfinger on March 23, naming a special investigator to get the case completely outside of the normal routine. One would assume that "the clock starts over" with a brand new investigator.
-- he should be publicly released for lack of evidence and the states attorney stating they're dropping all investigations. --
The decision will most certainly be very public. Meanwhile, the state has treated him fairly while the media has crucified an innocent man.
I have a question. When GZ met with investigators at the scene the next day to go over what happened, did he have a legal advisor with him? Even if I was 110% innocent I’d bring a lawyer along.