Try activating with Microsoft ToolKit, and just add the leaked KMS key to it. Worked on a test computer for me You will find all the things needed on the forum
It is six months for KMS activation, to be precise. Off-topic: Does anybody notice the forum pages loading very slow? And, sometimes don't load at all.
Okay, now it says I am activated. So it seems that it's true what people have been saying: just keep trying and it will work. I definitely didn't try 100 times though, maybe 30-50 tops. However, I don't think you should be spamming - there's plenty of time to activate and everyone mashing the button at once certainly won't help. I was really worried too, because it was saying I had a generic key. Now all is well, though.
Pretty much works the same way of KMSPico IIRC. You'll have to find another key to activate through that KMS server once that one expires.