For my domain and in my experience (deeply embedded firmware on custom mixed signal ASICs) ramp up time is around 6 months to break even, a year to be modestly productive, and 2 years to be actually productive (because you understand enough of the domain to contribute at a cross-functional, system level).

Hiring managers need to look for good employees as well as people who solve their problems, and whoever joins the team is going to need months of training to be productive.

Someone who jumps ship after a year is...not entirely useless, but was certainly a waste.

Perhaps it's different in other domains.