The more I think about that, the more I realize it's the wrong question.
I don't know if I should be controlling it (since the user probably expects it to be whatever they are used to). But, if it varies by platform, I'd like to know that. Perhaps there should be an env variable for blink speed. That would allow something like:
input:focus {
animation-duration: calc(4 * env(caret-blink-rate));}