This article will guide you in setting your customer logins to auto extend if the customer is active. 

Requirements: A Master Admin login or a staff type with access to the following permissions:

• Setup: Can access setup
• Setup: Can access general settings

Setting Logins to Auto Extend

To locate this setting, go to; Setup > General Settings > Other Misc> Auto Extend Expired Logins If Active.  If the customer login is about to expire then it is automatically extended.