Constant framenode_runtime::constants::time::MILLISECS_PER_BLOCK

pub const MILLISECS_PER_BLOCK: Moment = 6000;

Since BABE is probabilistic this is the average expected block time that we are targetting. Blocks will be produced at a minimum duration defined by SLOT_DURATION, but some slots will not be allocated to any authority and hence no block will be produced. We expect to have this block time on average following the defined slot duration and the value of c configured for BABE (where 1 - c represents the probability of a slot being empty). This value is only used indirectly to define the unit constants below that are expressed in blocks. The rest of the code should use SLOT_DURATION instead (like the Timestamp pallet for calculating the minimum period).

If using BABE with secondary slots (default) then all of the slots will always be assigned, in which case MILLISECS_PER_BLOCK and SLOT_DURATION should have the same value.

https://research.web3.foundation/en/latest/polkadot/block-production/Babe.html#-6.-practical-results