The activation queue length aims to answer this question:

“If I were to deposit ETH into the beacon chain contract right now, how long will I have to wait before my validator becomes active?”

To answer this question, first we need to consider how long it takes for the Beacon Chain to recognize a deposit to the beacon chain deposit contract on the execution layer. This used to take a minimum of ~14 hours (Reference) but with the Pectra upgrade, specifically EIP-6110, deposits are now processed on the same block they appear. Now with the gas limit per block being the only limitation for deposit processing, compared to the hardcoded 16 deposit limit per block, deposits can now be recognized almost instantaneously.

To get the amount of time it will take to activate these validators who have made the deposits, we need to get the churn limit per epoch which is based on the stake of currently active validators. The churn limit caps the number of validators that can be activated per epoch based on stake. We take the total stake balance of recognized deposits and divide that by the churn limit to get the number of epochs it will take to activate the stake and the related validators (i.e. clear the queue).

We then take the minute equivalent of this number of epochs (6.4 minutes per epoch) and add it to amount of time it takes for the Beacon Chain to recognize deposits to get the full activation queue length in minutes.