If you are entering a new niche, you should do market research first. The goal is to understand what they want and need BEFORE investing time into developing a solution that may not resonate.
This "interview method" allows you to quickly gather data from members of your niche. You can use this data to identify patterns and hypothesize a solution based on that feedback. This alone can save you months or even years of wasted effort chasing the wrong offers.
Best of all, you can sell your services to these folks once you have completed your research and designed a solution, so there is no wasted effort. You are doing R&D, marketing, and sales all in one.
You want to shoot for about 20 interviews, but the real indicator of success is when your interview subjects start repeating themselves and you can predict what they will say. This level of knowledge allows you to create deep rapport and shows that you've uncovered real patterns in your niche.
Keep in mind that you won't be able to get this type of information from statistics or articles on the web. You will be conducting in-depth market research and coming up with your own as a result of this process.
If you are starting from scratch, expect this to take 1-2 months, although you can complete this within a few weeks if you ramp up the number of requests you send out.
Make sure you keep accurate notes and ask all of the questions each time. This reduces the number of interviews you need to conduct and will help you finish faster!