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.
