Missy, welcome to the forum. It is possible to supplement your income mystery shopping. All the questions you have asked depend a lot on the shopper and the area of the country where the shopper is located. A more motivated, self-starter will make more money mystery shopping. The area of the country is important as well, because the more competition you have in your area the fewer jobs you will get and the less money you will make. How many hours you put in depends on how many assignments you take and what kind of assignments you take. Some assignments are very time-intensive and others are fast. The amount of money varies per assignment also. Some jobs pay more than others.
The way it works is that you sign up online with companies, providing your personal information and your SS#. You sign their independent contractor agreement, which usually includes a nondisclosure clause. After you have registered with a company, you will be able to search their job board to see if they have jobs in yours area and you can request those jobs. The more companies you sign up with, the more jobs will be available to you. For a lot of information, go to the New Shopper Area and read the top threads which explain in detail how to run a mystery shopping business.