I think I got U Miami's secondary about 2 or 3 weeks after the postcard, although I was in Boston, so it took a few extra days to get to me. When you do receive it, I think there's a deadline to send it back.
Last year it took 2-3 weeks to get a secondary. But, a secondary is not guaranteed at UM. They send you one only if they like your application. Call them just to make sure. They are starting interviews on September 13, so interview invites for the first 4 Fridays of the application year most probably already went out. Good luck!