The first retargeting message can be sent when the user has abandoned their cart. Meaning, when there has been no interaction for a specific amount of time. The timing of this retargeting message can be decided entirely up to you.
For example, imagine you decide that the first Text Message is sent 20 minutes after the cart was abandoned. If the visitor comes back on the website within 20 minutes after the cart was left, the timer resets to zero and runs for another 20 minutes until the Text Message is sent.