It's steps. E.g. if it's every 15 days, it at least pushes you to the point of automating it (HOPEFULLY) and the app managing it internally - that already helps against stupid shit like "someone put it in code/pushed a config file/... to a repo that later got compromised". Similarly, every X months is still a gain over keys sticking around many years. But yes, at the same time, if you get to have a reliable automated flow there is little reason to not run it with higher frequency.
Hm, not specifically. OAuth2 specifications and documentation sort of address the motivation for Refresh Tokens at least (and are widely written about in blog posts etc) - and I think the security recommendations documents now strongly push for Refresh Tokens. For the benefit of automated refresh one could also pull the Let's Encrypt arguments as "similar enough" and widely recognized as good practice.
