If the customer will stay on your website long time before he place an order. He will be logout. He need to login again to complete the order. It maybe cause some inconvenience. And you want to keep the customer logged in long time.
Maybe you know that the time for logged based on life time of cookies and session. So you can change these configuration in admin page to get your idea.
Change cookies life time for frontend ( customer user ): Admin -> System -> Configuration -> General -> Web -> Session Cookie Management -> Cookie Lifetime.
Change session life time for backend ( admin user ): Admin -> System -> Configuration -> Advanced -> Admin -> Security -> Session Lifetime.