I voted for every 6 months but I think it is not a great idea, I don't know why, but yea, it's not that good of an idea. I think you should keep it every 6 months but a limit of how many times they can change thier username. So that every time they change thier username, a count is placed that keeps on lowering the number of username changes they have done.
I hope I didn't confuse anyone....
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