The criteria and metrics can be classified according to its type and time span. Regarding the type, we can either evaluate these campaigns "Quantitatively" or "Qualitatively". Quantitative metrics may include "Sales Volume" and "Revenue Increase/Decrease". While qualitative metrics may include the enhanced "Brand awareness, image and health" as well as the "relationship with the customers".
What kind of commission do they offer? One-time commissions or recurring commissions? For example, many programs pay you one time for sending a customer. On the other hand, some programs like membership sites or SaaS (software as a service) programs will pay you a commission as long as the person you referred is a paying customer. Recurring commissions are great when you can find them!
Now most affiliate programs have strict terms and conditions on how the lead is to be generated. There are also certain methods that are outright banned, such as installing adware or spyware that redirect all search queries for a product to an affiliate's page. Some affiliate marketing programs go as far as to lay out how a product or service is to be discussed in the content before an affiliate link can be validated.
How much will you make? I think my best month with Google AdSense was almost $5,000 over the last ten years. That amazing month blew my mind since it was actually near the beginning of my blogging journey. When you go from making zero to $5,000 in a month, that will rock your world. For me, it also got me even more excited because I knew there were other ways to monetize.