We are not told specifically how long the jury take when deliberating Tom’s case. However, we do know that they took long enough for Atticus to feel that he had made a difference in that attitudes towards the way the justice system treated black men-
That jury took a few hours. An inevitable verdict, maybe, but usually it takes ‘em just a few minutes.
Atticus realises that his efforts to prove that Bob and Mayella Ewell were lying, and that Tom Robinson could not have committed such a crime due to his physical disability made some of the jury consider, however briefly, the true nature of those involved rather than just the color of their skin. He may not have been able to get the jury to accept the truth, but they were closer to seeing it than any jury before them.
There is not a definitive answer to this question in the book. We do know that the jury took longer than usual to deliberate, but we also know what the outcome will be in the case. Atticus has done everything he could do and proved that Tom was innocent, but because he is a black man everyone knows that he will be found guilty.
Jem is convinced that the jury will come back with a not guilty verdict. He is so convinced that Atticus has done what he had to do to prove the innocence of Tom. Jem is devastated by the outcome of the jury's deliberations. There was no way in the town of Maycomb, that Tom was ever going to be found innocent. I think the jury took longer than usual, because they were a group of people that knew they were going to find him guilty, so they took their time coming back. It was almost as if they were exerting their power over Tom and enjoying every minute of it. Some of the people on the jury did think about the trial. They did have a harder time coming to terms with the fact that Atticus proved he didn't do it. This was the first time in Maycomb's history that some of them struggled with the decision to convict Tom.
The whole story goes to show the true nature of people's hearts. Some of them were good and wanted to do what was right, while the others were still stuck in the old way of doing things. These issues are still relevant to this day, just not as in your face as it was back then.
There are a few pieces to this puzzle that can help to round down the number of hours that the jury takes to deliberate. For example, at the end of chapter 18, Judge Taylor says, "It's gettin' on to four," which means it is around four o'clock in the afternoon when Tom Robinson takes the stand. Tom Robinson's testimony and cross-examination could have taken less than an hour, but the timeline is vague because Scout and Dill go outside to take a break and end up talking with Dolphus Raymond for a little while. When Scout and Dill go back to the courthouse, Atticus is beginning his final closing statements to the jury. If his final arguments take about twenty to thirty minutes to give, without taking into account that Scout and Dill missed Mr. Gilmer's final arguments, the time would be around 5:30-6:00 pm, which seems perfect because Calpurnia shows up looking for the children to feed them dinner.
The kids go home to dinner with Atticus at that time and they have a calm, sit-down dinner. The jury is deliberating while the family eats; and, they probably started deliberating right after Atticus's closing statements around the time Calpurnia showed up around 5:30 or 6:00. As they eat, the kids ask their father if they can go back to the courthouse for the verdict and he consents. Now, if dinner lasted thirty minutes or so, everyone could have been back to the courthouse between 6:30-7:00 pm.
While everyone waits for the verdict, Jem asks Reverend Sykes what time it is and he tells him, "Gettin' on toward eight" (209). The next time given to the reader is when Scout says the following:
"The old courthouse clock suffered its preliminary strain and struck the hour, eight deafening bongs that shook our bones. . . When it bonged eleven times I was past feeling: tired from fighting sleep, I allowed myself a short nap against Reverend Syke's comfortable arm and shoulder" (209).
Since Scout does not mention the clock striking twelve, the verdict is delivered sometime during the hour of eleven and before midnight. Therefore, the jury most likely was deliberating between the hours of 5:30-11:30--or at least before midnight-- which accounts to about 6 hours.