In that "planning" that catchlights mentioned, it is more like researching and understanding what people need / will need, rather than just shooting what you like and hope someone will buy it. Probably, just like other projects you of course have to plan, manage your time resources etc.you need to work very hard,
planing, sourcing, shooting, post processing, keywording, uploading,
than start this cycle again and again,
after two to three years, you should have thousands over files if you spend at least 10 hours a week,
than able to see some returns.
hope this help.
hope that adds