How long does a post-quantum migration take?

Years for most organisations, which is why starting early matters more than the exact deadline. The change touches every place that agrees keys or signs data: websites, VPNs, SSH, e-mail, internal applications, certificates, hardware security modules, smart cards, machines and their firmware. Much of it depends on suppliers shipping post-quantum support, and on your own replacement cycles: a firewall bought this year may still be running in the next decade, and an industrial controller far longer. Past changes give a sense of scale: retiring SHA-1 and the old TLSTLS: Transport Layer Security. The encryption that protects most internet traffic, such as HTTPS. versions took the industry many years from the first warnings to the final switch-off. A small company that runs mostly cloud services and up-to-date software may get much of the way through ordinary updates, inside a normal refresh cycle. A company with in-house software, long-lived devices or its own certificate authority should expect a programme spread over several years. The Y in Mosca's inequality is your own estimate, so write it down with its reasons.

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