I don't work for the courts, but I interact with courts (from MDJ/County/State/Federal) every day.

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Jury selection, for the most part in PA, is a demand for 8 or 12 people.

Out of a pool of 50, on average 10-15 will have personal conflicts (aka they know someone directly involved - be it attorney, ptf/def, arresting officer).

This takes the jury pool to 35.

10-15 will be, for lack of the better word, unreliable (i.e. don't trust the police, have a really bad record, which believe it or not knocks your credibility)

This takes the jury pool to 20. Depending on the type of case, the attorneys will weed people out by age/race/sex/assumption of political party, bringing the realistic jury pool down to ~15 people.

The reason for the highlighted part, is some of these remaining ~15 people, frankly, will just not show up. So if you can go, they desperately want you to go to make the jury pool more diverse.

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I've seen jury selection as fast as a few hours, up to a week.

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EDIT: This is a vast generalization by the way. Jury pools are not limited to 50 people, but the numbers usually even out to be the same percentage wise.