ditto what subiesailor said.
In addition, I tend to agree it's not likely, although not impossible, that two sets of axles would have the same faults as the originals, leading to the same "clicking" symptom, unless they are really poor fitting aftermarkets.
Also, the fact that the clicking starts only after the car has been driven for 15 or more minutes could suggest it's something else.
After the first and second replacements, it appears that the clicking didn't show up immediately. Were there any times immediately after the axles were replaced that the car was driven for a longer time without the clicking showing up? In other words, did the clicking not show up in the first two days because the car wasn't driven far enough to cause the clicking, as it would have before?
I ask because there's another possibility if the tire are "grabbing" when making tight turns, and that's a failing center viscous coupler (part of the AWD transfer mechanism). When these fail, they often do so in a way that causes the wheels to skip/hop/shudder in tight turns, but not when going straight.
The viscous coupler works on the basis of heat generated inside its housing, so the time lag (e.g., 15 minutes of driving) before the clicking starts is consistent.
One common cause of this is tires that are not properly inflated or have different circumferences. (See: Tire circumference spec confirmed by Subaru (TechTips)
) All four tires should be the same brand, model, size, and have the same tread wear (tread depth). Even a tread depth difference of as little as 1 or 2 thirty-seconds of an inch can cause problems with the viscous coupler. If left long enough, the coupler, which might initially "self correct" when it cools, will become permanently faulty and even correcting for unequal tires won't correct the problem.
The clicking sound might just be the sound made by the drive train as the tires hop/skip/shudder.
Not sure that this is the problem, but it would be consistent with your description.