For example, after the first message is sent, the second message is sent 12 hours later. Is there any way I can change the gap duration?
Each message can be configured separately to be active for a certain time period. To configure a message, go to your Engage portal, then under Campaigns, select your Campaign> message to be configured> (gear icon in front of your message/return offer) Edit.
A page will open up, giving you options to set up day, start and end time and frequency of messages for the subscribers. Make sure to save the changes once done.
Further, to set the period of time between the delivery of messages to the same subscriber, configure the cool-down period for your campaign. To do so, go to the gear icon in front of your campaign> Edit >Advanced.
You can also preview the messaging intervals by clicking on the preview tab (eye-shaped tab right next to the reports tab).