@Hustus may need to elaborate on the way it works but the way I read it and using these assumptions:
Regardless whether you're premium or not, if you have $20 million in the bank, you can earn maximum interest of $20,000 every 30 minutes which equates to $40,000 an hour.
This assume it only works while you are actively playing on the server.
Assumes you play 4 hours a day.
$40,000 x 4 = $160,000 of interest per day.
$1,000,000 / $160,000 = 6.25 days.