The current estimate for the upgrade can be found at [1]. It's not a regular upgrade that happens at a specific block height, hence the variance. Instead, the consensus is flipped over from PoW to PoS at a specific total difficulty, also known as mining difficulty. One of the reasons is to avoid miner attacks around the merge event.
It depends on a host of factors, so it's hard to predict exactly.
As an example: it depends on the rate at which existing Proof of Work miners move the chain forward. We can make an estimated guess based on the current hash rate, but miners may begin to drop off early and try to "beat the rush" to sell their used hardware before everyone else, so the hash rate could drop more than anticipated.
>The target date is September 15, 2022, but this estimate might have even a week of error.