Re: Extension to enforce admin user password resetting every X days.
Hi, this is something that Magento Enterprise does so if you have access to that codebase that might give you some inspiration. I haven't come across third party extensions that do that myself.
I'm pretty sure there's research out there now that suggests that forcing users to change their passwords regularly like that leads to weak passwords because it's common just to add a number on the end.
I'd suggest these alternatives for your consideration: