"Remember Me", at least as far as FTM is concerned, keeps you logged in for a month. (It's only a month because of a mandatory[?] 30 day life limit on the cookies that store the pertinent info.)
Apropos of that, I just received an announcement about
Ghostery Lite which mentions, among other things, the following...
Apple recently announced that with the macOS Mojave release (sometime in late 2018), Safari will take a more aggressive stance on privacy and security. This release will include:
Cookie partitioning with Storage Access API requires user consent for 3rd party cookies on each website and purges all cookies after 30 days
So as far as
Safari goes, beginning with Mojave, "Remember Me" will be limited to 30 days on
ALL websites.
I hope there's an override option for that restriction.