We do this for our company. When people opt in, we add them to the once every 5 day option. In our subscription center we allow them to select daily, weekly, every other week, monthly, or every other month emails. Based on what they select, we have flow actions that add them and subtract them from an okay to send list. For example if some one is subscribed to weekly emails, and they receive an email, we have a listening campaign that says if they are subscribed to only receive emails once every week, if they are sent an email put them in the not okay to send list, wait 7 days, and then add them back into the okay to send list. Hope this helps.
Josh