Generally when you are talking about Obliterate and waiting for KM, you're talking about 2H Frost, the fact you brought it up like you did pretty much shows you didn't read the thread, or you don't know the difference. For your sake, I'll assume the former.
Anyway, as I said above, you shouldn't be waiting too long as you'll be wasting future KM proccs or could waste Runes/RP.