How to set the number of days until subscribers get logged out

The WordPress default is 14 days if "remember me" is checked during login.

Adjust how long a user stays logged in before WordPress logs them out. This plugin helps: https://wordpress.org/plugins/wp-persistent-login/

If you are using the Leaky Paywall login form, you can adjust any of the arguments of the wp_login_form function using the filter leaky_paywall_login_form_args.  

Set Remember Me to checked by default