Epoch

An epoch is a unit of time when validators of the network remain constant. It is measured in blocks:

Both testnet and mainnet have an epoch duration of 43,200 blocks. Ideally epochs last about 12 hours, since blocks are created every second (in reality, they take slightly longer to be created).
You can view this setting by querying the protocol_config RPC endpoint and searching for epoch_length.

Note: Nodes garbage collect blocks after 5 epochs (~2.5 days) unless they are archival nodes.

Example:

JSON
HTTPie

{
  "jsonrpc": "2.0",
  "id": "dontcare",
  "method": "EXPERIMENTAL_protocol_config",
  "params": {
    "finality": "final"
  }
}

http post https://rpc.testnet.near.org jsonrpc=2.0 id=dontcare method=EXPERIMENTAL_protocol_config \
  params:='{
    "finality": "final"
  }'

Example Response:

{
    "jsonrpc": "2.0",
    "result": {
        "protocol_version": 44,
        "genesis_time": "2020-07-21T16:55:51.591948Z",
        "chain_id": "mainnet",
        "genesis_height": 9820210,
        "num_block_producer_seats": 100,
        "num_block_producer_seats_per_shard": [
            100
        ],
        "avg_hidden_validator_seats_per_shard": [
            0
        ],
        "dynamic_resharding": false,
        "protocol_upgrade_stake_threshold": [
            4,
            5
        ],
        "epoch_length": 43200,
        "gas_limit": 1000000000000000,
        "min_gas_price": "1000000000",
        "max_gas_price": "10000000000000000000000",
        "block_producer_kickout_threshold": 90,
        "chunk_producer_kickout_threshold": 90,

// ---- snip ----
}