Sort:  

I created an infographic back in November, 2016. Here's what I posted at the time:

image.png

Obviously, I'm hazy on details, but maybe interest accumulated and paid out once per month? No time now, but maybe I'll check later to see if I can find any payment transactions from the time period.

Update: Found one

maybe interest accumulated and paid out once per month

Right!

#define STEEM_SBD_INTEREST_COMPOUND_INTERVAL_SEC (60*60*24*30)

Once per 30 days and only if the SBD balance has to by modified.