The ledger at a point

client.ledger_state_at(point) acquires the ledger as it was at point. Every ledger query on it then answers for that point, however far the chain has moved on:

import pyogmios

with pyogmios.Client("ws://localhost:1337") as client:
    tip = client.network.tip()
    with client.ledger_state_at(tip) as ledger:
        epoch = ledger.epoch()
        parameters = ledger.protocol_parameters()
        pools = ledger.stake_pools()
    print(f"at {tip}: epoch {epoch}, {len(pools)} pools, protocol {parameters.version.major}")

With asyncio, it is async with client.ledger_state_at(point) as ledger: and await ledger.epoch().

Use it when several answers must agree with each other: queries on client.ledger_state each answer for the tip at that moment, and the tip can move between two of them.

How far back

Nodes keep only the recent ledger states, roughly the last 2160 blocks (about 12 hours on mainnet). Acquiring an older point, or one not on the chain, raises AcquireLedgerStateError. An acquired state can also become too old while you hold it; queries then raise AcquiredExpiredError: acquire it again.

Its own connection

In Ogmios, an acquired ledger state applies to every query on the connection. So ledger_state_at opens a connection of its own, and the client's other queries keep answering for the tip. When the block ends, the state is released and the connection closed.