I think stress and care are the biggest factors in how long a genetic lasts. I ran my WW for over 4 years, and only the first batch of clones came from the seed mother. After that I lost track of how many generations deep my clone taking went. I replaced the mothers about 1-2 times a year depending on various things. Until last summer I had always takken good care of the mothers, and the clones, and I never felt there was much if any degrade over the time I ran it. Last summer with my accident my plants had some rough weeks, I lost one mom all together and the other took a beating. After that the plant never really seemed quite the same, and I just recently harvested the last of it after letting the mother go.
Now contrary to my experiances with my WW. I got a cut of some Bubba Kush a while back, and ran it a few times then that was the mom I lost at the accident. I got more cuts of it a few months later from the same guy, and same mom but they grew very different, and kinda scragly. They tasted, and smelled the same, and were just as frosty and dank but the bud structure had changed. So the guy asked me if I had noticed this which I had. The only thing we can figure out is that in the year between when I got cuts the mother had taken a ton of abuse. It had been moved many times, cut off, frozen, dried out, alll kinds of stuff.
So like I said at the beginning I think plant care, and stress or lack of make the biggest impact on how long genetics last. Some people WON"T grow from clones, or WON"T clone more then 1-2 generations. But come on some of the dankest genetics in the world have been around for many many years in clone only forms.......
srry for the long post, I'm toasted, and just started rambling
peace!