If you're hoping to achieve a durable paint job with Rustoleum enamel, then you'll need to use a hardener. The amount of hardener you'll need will depend on the size of the area you're painting as well as how many coats you plan on applying.
For example, if you're painting a small area and only plan on applying one or two coats, then you might only need 1/2 pint of hardener. On the other hand, if you're painting a larger area or want to apply multiple coats for extra protection, then you might need 1 pint or more of hardener.
To be safe, it's always best to consult the Rustoleum enamel instructions before beginning your project. That way, you can determine the exact amount of hardener needed for your particular application.