Probably half can complete them in one day and a few more in 2 days with preloading certain characters. And then a few more that are only a couple of characters away from being done with ops.
Okay, but you forgot the most important bit: how many of these 7-10 guilds are going to miss out on 6/6 just because of Chieftain?
We disagree on the amount of notice (though I think it should be more, I don't think it needs to be 6 months, which assumes that out of 50 people in a 6/6 ROTE guild not one single player has even started a farm -- a ridiculous assumption), but the number of your guilds that miss 6/6 just because of Chieftain is a fact that you can find out. I will take your word for it. How many guilds will miss 6/6 based on this alone?
If the answer is zero, doesn't that make your argument that 6 months notice is needed a little silly?