My First Time (hiring ad agency)

I decided to pay for ads and a small agency to do it for me and I am SCARED.
I will use the ads to increase the number of people on my list, hopefully, with people that will eventually buy the online course, I offer – one day.
BUT it could take months before some of them will convert to paying costumers, if at all.
1. What is considered to be a successful campaign? (How much I spent vs how many people joined my list)
2. How long do I keep the campaign before I pull the plug or change the ad agency? And what are the signs I should do that?
3. Any tips for working with the agency? What to ask so I will learn from this process?