It is an interesting one - here is the original source (and there were many more mentioning 300 days).

https://community.letsencrypt.org/t/dns-authorization-lifeti...

It is from 14 June 2016 (a few months back), @pfg states "The CA/B Forum is currently developing new rules for domain validation and is probably going to settle on a validation period that is significantly longer than the 300 days currently in use ..."

Is there an authority to say which way it will go?