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When two blocks are mined and propagated at similar times times causing a fork, how long would a node store a stale chain until considered orphaned?

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How many blocks (work) are needed on chain I for chain H to be discarded?

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Bitcoin Core will never discard reorged blocks, until they are pruned (which happens regardless of whether the blocks are in the active chain or not).

Only one chain is considered "active" at a time though. The rules are:

  • Only consider fully valid chains (i.e. if a block contains an invalid signature or anything else illegal, it is ignored, together with all its descendants).
  • Among those valid chains, only the one whose tip has the most accumulated work (roughly equal to the sum of the difficulties in the chain) is considered.
  • If there are multiple valid tips of equal accumulated work, the one whose last block was received first is considered active.

All other chains are considered inactive, but if they get extended, they may end up becoming active again.

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  • Thank you. By pruning, do you mean the optional pruning mode to save disk space?
    – Yazid
    Dec 4, 2022 at 18:21
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    Yes, indeed. It just removes all old blocks past some threshold of age, regardless of whether they are part of the active chain or not.
    – Pieter Wuille
    Dec 4, 2022 at 18:30

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