Avis - Odds of an Upgrade

What are the odds that they will give me a higher class than the one I paid for (upgrade) at Miami International Airport?
It's a 9 day rental in november for the premium class.

Fleet management, time of day, and charm may all have to do with it. Do you have status with Avis? Otherwise, it's a crap shoot. If you keep your expectations in line to receive the car that you booked and paid for, then you won't be disappointed (except in the rare case they downgrade you). Any upgrade would just be icing on the cake.

Rented once in MIA, and received an upgrade. But I'm probably more the exception than the norm on what to expect.

I don't think you should expect any upgrade from Premium. You might expect an upgrade to Premium from Full Size, if you have status, but probably not without status.