I don't generally comment on Bearing Threads, but if they're only lasting that long, there can only be 3 Reasons...
Your not fitting them correctly, something is damaged somewhere else causing undue loads on them or They're really bad Quality.
I sell bearings for a living of every type and description you can think of and in my life only the first 2 apply. I don't sell cheap bearings
I would check everything really well, maybe the hubs have worn and this is causing them to sit wrong and this then is leading to premature failure...