After sending off a conversation starter to your contact (the first message tied to a Smart List), you will see a cool down period reminder at the bottom of the conversation panel.

This reminder lets you know the date of when you can send a follow-up message to your contact. The default is 5 days from the first message, but this can be modified by your organization. The cool down period is designed to prevent spamming or overwhelming your contact with too many messages in a short period of time.