Based on the length of the initial expansion patch. 9.0 is the longest, non-endgame patch in the game’s history at 218 days. That’s only four days shorter than WoD’s 6.0 and 6.1 patches combined and half of WoD’s final patch, aka the second longest patch in the game’s history. First prize in that department goes to MoP’s 5.4.
Let’s do a little math here. I’m using this chart as my guide.
I’m excluding Vanilla because it was a totally different patch system with 12 major content patches, and Shadowlands past 9.1 because it isn’t finished yet.
The average x.0 patch is about 108 days, and the average x.1 patch is about 120 days long. That puts the x.0 patch at about 111% longer than the x.1 patch on average. Shadowlands has already surpassed that metric, but we can possibly forecast how long 9.1 will last. This gives us an estimate of about 242 days. That puts the end of 9.1 around February 26, 2022.
The average length of x.2 patches is about 175 days or about 146% of x.1 patches. That would put the end of 9.2 at around 353 days. I highly doubt they’ll do that, since that would put the entire expansion at 813 days, but it’s not outside the realm of possibility considering BfA’s final patch pushed it to 833 days. Average expansion life is about 724 days.
Can patches be shorter? Of course. These are just averages, not concrete. All’s I’m doing is speculating, and the further we get into this expansion, the more we can extrapolate.