How often do I need to show terms and conditions to customers?

A website's Terms and Conditions (TC), or Terms of Service, attempt to or serve to create an enforceable agreement between a website operator and varying classes of users who may visit or functionally interact with the website. As the website operator, you want to ensure that in the event you wish to enforce the TC's obligations against a user, the user is less able to raise defenses against your claims (e.g., the user never read, and thus could not agree to, the TC). Also, you want to ensure that in the event a user wishes to bring claims against you for your own violation of the TC or associated privacy policies (a notorious source of litigation from California plaintiffs), you can enjoy the liability limitations which you hopefully built into the TC. Because visiting your website and making purchases would be discrete, separable transactions, you will want the user / customer to be affirmatively agreeing to the TC and acknowledging that he/she has read and understands them. Because your TC should be periodically revised and updated to best-protect you, you will want to ensure that any transaction occurring after that fact between you and a user is invincibly pursuant the updated TCs.