There is a setting that saves the day delay for a specific drug for a specific patient. When you edit the SMS reminder for that patient’s medication, make sure you tick the “Link with future dispense of same drug” option.
This option will remember the day delay the next time you dispense this medication for this patient. However, this setting only applies when you have it set so the patient automatically receives repeat reminders i.e. the setting highlighted below.
If you do not have this option enabled, you have to manually create the SMS repeat reminder and manually set the delay.