I don't think you are actually paying attention. I QUOTED your statement of "after some time" (which is why it was in quotes.
After some time is NEVER if you have a standard, unmodified installation and don't delete cookies.
What you are experiencing suggests that either you have messed around with the code, in which case, all bets are off and it could be doing anything, since we have no idea how you have screwed up the code or that you are deleting your cookies on your local browser - which will, indeed end your session.
Firstly my bad, I replied to this having seen the Forever login title, missing that it was for SMF 2.0 where I am running 1.1.19.
Whilst I have multiple mods, and have had to make some minor changes in the past to mod code to fix issues with my site, none of this has been done recently and certainly not since the recent update. The forever login has been working fine, though recently, not sure if it coincided with the security update I'm afraid, I have noticed this issue happening on my devices connecting to the forum.
I have made no browser config changes, no code changes but yet I see this happening despite your insistence. I merely posted to confirm that someone else was seeing the same issue as the OP. Not a problem for me to keep logging in, just thought I would try and help as there may be an issue.
Just because something should work does not mean there is not a problem, especially with such a complicated bit of code as SMF, it is always far to easy to miss something.
That said, I could not code anything as complicated as this and concede to your superior knowledge of your own product for which I thank the entire dev team for their continued efforts it is much appreciated.
I will just resign myself to logging in more frequently and bow gracefully out of this conversation.