Erica, it all boils down to how much time you are willing to spend on it. When you are first getting started you will spend a whole lot more time registering with companies and checking their job boards than anything else. You will need to regularly check the job boards of the companies you are signed up with to see if they have posted jobs that you can self assign or request.
Until you determine what companies actually shop your area and when they are likely to post you could readily spend the better part of a day doing this and find no jobs at all. That is why I recommend that folks register with Corporate Research International, Bare, Market Force blue portal and Maritz. These 4 companies have nationwide clients so can provide you your initial jobs to see if you even like mystery shopping. A daily job check of their board should take you under an hour. Over time gradually add new companies and if you see no jobs posted with them over a few months, stop checking their job boards regularly but do allow them to contact you by email should jobs subsequently appear in your area.
How much can you make? There are folks who regularly make upwards of $20k doing this, but they are much more willing to drive much greater distances than I will and purchase expensive video equipment to do video shops. We all find what works for us. My particular needs from mystery shopping are supplemental income so by now I spend less than an hour a day average checking for work and generally earn between $10k and $15k per year, approximately 1/3 of which is reimbursements that are useful to me such as oil changes, groceries and dining out.
As a general rule, mystery shopping works best as supplemental income rather than making any pretense of 'earning a living' at it. Because of pay cycles and purchases required to get receipts to be reimbursed, this is not a business to 'try' if you already are cash strapped. This month, for example, the work I have done or have scheduled will have me advancing about $350 of personal funds that will be reimbursed at the end of February or into April. Fees for those jobs will be paid at the same time that the reimbursement is sent. It is possible to work for faster paying companies with smaller out of pocket costs but they generally are jobs with a value of under $15 that only make sense when you can run a route of a number of them and still get the reports in before bedtime.