This is usually expressed as the long term value (LTV) of the customer. It costs X to get a customer but once they know you they may use you again, refer you to

This is usually expressed as the long term value (LTV) of the customer. It costs X to get a customer but once they know you they may use you again, refer you to others, etc. and take Y days/weeks/months to repay you for customer acquisition costs. For instance with magazines people used to subscribe for 3 years on average which is why so many companies love subscriptions. They have enough inertia and friction that people just keep paying in many cases.