How far can I push lock_time?

[ltcltcltc]

Is there a limit to how late a lock_time can be set to?

If not, suppose I send out a transaction that's time-locked to 1 day/month/year/decade into the future. To what degree of certainty will my transaction be eventually be included in a block? In particular, do later-time-locked transactions need higher transaction fees in order to keep the same probability of being included in a block at t ≥ lock_time?

Thanks!

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[Charles-Tim]

If the transaction is broadcasted and the transaction fee is high and the transaction is not getting any confirmation, all that can be done at that time is to use child-pay-for-parent to increase the chance of the transaction to get confirmed. I will advise anyone that wants to use time lock to use a high fee rate.

[hosseinimr93]

Is there a limit to how late a lock_time can be set to?

The maximum value you can set for the nlocktime is 4,294,967,295 (0xFFFFFFFF in hexadecimal system)
That's equivalent to February 7, 2106 6:28:15 AM.

If not, suppose I send out a transaction that's time-locked to 1 day/month/year/decade into the future.

it may worth mentioning that nodes reject any transaction with a locktime into the future.
If you have a transaction with a locktime into the future, you have to wait until its locktime and after that you will be able to broadcast it.

In particular, do later-time-locked transactions need higher transaction fees in order to keep the same probability of being included in a block at t ≥ lock_time?

We don't know how much fee rates will be required for getting confirmation in the future.
If the fee is low, as mentioned above by Charles-Tim, you will need to do CPFP after broadcasting the transaction in the future.

[achow101]

If not, suppose I send out a transaction that's time-locked to 1 day/month/year/decade into the future. To what degree of certainty will my transaction be eventually be included in a block?

Your transaction is basically guaranteed to never be included in a block if all you did was broadcast it before the locktime was reached.

Nodes do not accept or relay transactions which cannot be included in the next block. If you broadcast a transaction with a future locktime, basically all nodes would drop it. Besides that, nodes will also evict transactions that have been unconfirmed for too long (typically 2 weeks as that is the default in Bitcoin Core). If you had a transaction with a timelock greater than 2 weeks in the future, and it was somehow accepted into some node's mempool, it would be dropped before the timelock was reached.