My guess is that it's not like the delay on getting other Tesla cars that's more-or-less continuous, so whenever you order there's pretty much the same wait. I imagine that most people who ever will get the 3.0 upgrade ordered it soon after it became available, and the 6 month number is just the time to run through the one-time-only backlog. So if you wait three months from now to order, it'll be a three (or maybe four) month wait, not six.
Another way to think about it is to ask how many other people will be ordering in the next few months? Not many, I'd imagine, so the backlog should go down at nearly one month per month.
I ordered maybe 10 days after it became available with just this in mind. I wanted to be toward the end of the line, because my existing battery is in decent shape for having 100K miles on it, but I also wanted to get the upgrade while they were still in continuous production so that I was able to get it at all, because I have a feeling that five years from now it'll be mighty hard to get.