The deal runs all day on Monday at participating Sonic restaurants and there's no limit on how many you can buy (except for if they run out of supplies).
Both hot dogs normally sell for around $2.
The All American Dog is a roughly 6" hot dog topped with relish, ketchup, chopped onions, and yellow mustard, while the Chili Cheese Coney is topped with chili and melted cheddar cheese (I recommend adding onions and mustard to it for the full Coney effect).