I think that trying to predict wait times by looking at historical times is probably not going to work.
It seems like what's happening is that Tesla can only do them so fast, and the limit is the number of spare ESSs that they have. Imagine that they only had one extra ESS. Then they'd take that one, upgrade it to a 3.0 battery, ship that off to a service center and install it, take the old battery from that car and send it back to Tesla for an upgrade, repeating the cycle. With one spare ESS, they're limited to one upgrade per cycle which probably takes a couple of weeks, I'm guessing. With 2, they could do 2, etc. until they start running into the limit of how fast they could process them at Tesla. Since 1.5s and 2.0/2.5s have non-interchangeable ESSs, they run on separate cycles (and evidence is that the 1.5 guys got it faster).
So, if that's what's going on, how long it will take you to get your upgrade is determined by how many people are ahead of you in line. My guess is that not too many people are signing up now (as opposed to when they first made it available), and in particular that people are signing up more slowly than they can process them. So, the longer you wait to sign up, the shorter time it is from signing up to getting your installation. I'd be amazed if you signed up in a year from now and it would then take another year to get your battery. Until, of course, they stop doing the upgrades altogether, which will eventually happen (if for no other reason than that they'll stop being able to get the new cells).