Stock JK OEM ball joint life expectancy varies widely. Some have gone out in 20,000 miles; some have lasted over 100,000 miles. Mine lasted 76,000 miles.
One variable may be people's opinions of what constitutes a worn out ball joint.
I measured the axial and radial play on mime at each tire rotation. That removes the subjectivity of the assessment.
Mine seemed to reach the 2/3 wear point rather quickly, then the final 1/3 of the wear took quite a while.
I replaced the originals with Synergy ball joints. They have zero play with 35,000 miles on them.
Here is what you need to know to check your ball joints as time goes by.
Radial (side-to-side) movement in the upper ball joint should not exceed 0.060" (1.5mm).
Axial (up and down) movement in the lower ball joint should not exceed 0.050" (1.25mm).
A dial indicator is the best instrument to measure ball joint play.
Jack the front tire off the ground.
Place a jack stand under the axle next to the jack.
Lower to put some weight on the jack stand.
Place a long pry bar under the tire, from the side.
Have someone pry the tire up and down while you watch the lower ball joint for up and down play.
Axial (up and down) movement in the lower ball joint should not exceed 0.050"
Have someone grab the tire at top and bottom to wiggle the top in and out while you watch the upper joint for side play.
Radial (side-to-side) movement in the upper ball joint should not exceed 0.060"
Even if the upper joint checks good, change both if the lower checks bad.