This thread is old, but I did not find the question answered elsewhere. Thus, here is my workaround:

• In file helpers/Authentication.php look for $cookie_expire =.
  
• Change the value following the equal sign to a larger value than a month,
  
• This leads to a later expiration date for the login cookie in your browser.

I am not proficient in php in any way, but looking at how the config.ini is accessed, you could also add the variable yourself:

• In file helpers/Authentication.php set $cookie_expire = \F3::get('cookie_expire_seconds');.
  
• In file config.ini add a line cookie_expire_seconds = 31536000.
  
• Do not delete the new line at end of the config.ini!
  
• This results in a expiration time of one year.