The activation queue length aims to answer this question:

“If I were to deposit 32 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 full 32 ETH deposit to the beacon chain deposit contract on the execution layer. This takes at a minimum ETH1_FOLLOW_DISTANCE + EPOCHS_PER_ETH1_VOTING_PERIOD, currently at around ~14 hours. Reference

The ETH1_FOLLOW_DISTANCE is currently set to 2,048 blocks with each ETH 1 block/execution layer block set at 14 seconds (SECONDS_PER_ETH1_BLOCK). Reference

The EPOCHS_PER_ETH1_VOTING_PERIOD is set at 64 epochs. Reference

These two combined form part of the activation pipeline concerned with processing the deposits.

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 currently active validators. The churn limit caps the number of validators that can be activated per epoch. We take the number of validators that have had their deposits recognized and divide that by the churn limit to get the number of epochs it will take to activate them all (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.