In chapter 23, Jem has a discussion with his father regarding the prejudiced jury during Tom Robinson's trial and mentions that the jurors made up their mind in a hurry. Atticus responds by correcting his son, and he tells him,

No it didn’t. . . . That was the one thing that made me think, well, this may be the shadow of a beginning. That jury took a few hours. An inevitable verdict, maybe, but usually it takes ‘em just a few minutes (Lee, 226).

When a black man is on trial in the prejudiced town of Maycomb, the black man is considered guilty before the trial begins, and, generally, the jury does not waste its time deliberating an open-and-shut case. However, Atticus valiantly defends Tom Robinson and proves his innocence. The lack of medical evidence, conflicting testimonies from the Ewells, and the fact that Tom Robinson is handicapped proves that he is innocent. Atticus's excellent, articulate defense influences the jury, and they deliberate for several hours. Atticus even tells Jem that one juror was arguing for an outright acquittal.

Even though the jury deliberates for a long time, Tom's conviction is predictable because of the racist nature of the jurors. Despite the unfortunate outcome, the extensive deliberation indicates that the town of Maycomb is making small steps toward racial equality and justice.

In Alabama in the 1930s, this case should have been decided and over with before an hour was up.  That should have been the case of Tom Robinson in this novel as well.  However, what is odd about it is that we find that the jury is conflicted.  They have seen the evidence (or lack thereof) and they know that the Ewells are lying.  However, society has taught them to be prejudiced, and this should be a open/close case.  It should be easy to decide.  In a later chapter we find that Mr. Cunningham's relative was the one to keep them out so long.  Of all people, the one man who brought his Sarum Bunch forward to harm Tom the night he was moved to the jail, Mr. Cunningham found respect for Atticus that night.  So his family all look at Atticus a little differently now.  That Cunningham was going to do what was right, rather than what society had taught him to do/believe.  If Atticus could do right, by golly, so could the Cunningham family.

It depends who you ask about the predictability of the verdict.  Every semester that I teach this novel I get ANGRY students.  They honestly believe that Tom is going to be let go.  He didn't do it.  However, they are surprised (and angered).  So I don't believe it's predictable.  Part of us knows he'll be guilty.  The other part prays that these people will do what's right. So we don't know until we read it.

Atticus knew that Tom was going to be found guilty. He was, after all, a black man trying to get a fair trial in the small southern town of Maycomb. Atticus knew exactly what was going to happen. He knew that he had proven without a doubt that Tom was innocent, but being a black man, he was already judged guilty. 

When the jury takes longer than usual, there is a silent sense of hope. Atticus hopes that the people of the jury will realize they are about to convict an innocent man. The members of the jury have already made up their minds, and are just talking in the jury room. The outcome of the verdict is very predictable. Everyone knew that Tom was not going to have any justice. Bottom line was he was a black man, and to the eyes of most of the people in Maycomb, that meant guilty. 

What is the saddest of all, is that a man like Bob Ewell, who is cruel and vindictive, gets some kind of justice, just because he is white. Being a black man in Maycomb, was a dangerous thing, and being accused of rape of a white woman, was a death sentence. Atticus had proved that there was no way Tom could have committed the crime, but just because he was a black man, the verdict was concluded long before the trial even began.