using winbox, go to: IP >> Hotspot >> User-Profile
change the keepalive timeout to the desired time. for example: 5d 00:00:00
this will keep the user logged in for 5 days.
But, be aware that if a user tried to login from another PC (and if no shared users , or more than one user is able to share the same account) then the user will not be able to login as the system will recognized that the same user is already online and using the service. At that time you have to disconnect the user manually or to allow multi-shared users (for example 2 users).
But .. why you are looking for that !!!
Regards,
Alaa