Moving from SHA-1 to SHA-2 took ~20 years - and that's the "happy path", because SHA-2 is a drop-in replacement.
The post-quantum transition is more complex: keys and signatures are larger; KEM is a cryptographic primitive with a different interface; stateful signature algorithms require special treatment for state handling. It can easily take more than 20 years.
Moving from SHA-1 to SHA-2 took ~20 years - and that's the "happy path", because SHA-2 is a drop-in replacement.
The post-quantum transition is more complex: keys and signatures are larger; KEM is a cryptographic primitive with a different interface; stateful signature algorithms require special treatment for state handling. It can easily take more than 20 years.