We have three major credit bureaus here in the United States: Experian, Equifax and TransUnion. Each is required to let you see your credit report once every 12 months. You can look at each one just once a year, and they might have different data on them, making this question a bit difficult to give a straight answer to.
Generally speaking, you should monitor your credit score closely. Fraud takes many forms, some easier to catch than others. If someone steals your credit card data, you'll know it pretty soon when strange charges start cropping up on your statements. But if someone steals your Social Security number and your mother's maiden name, say, they might start opening up lines of credit in your name in a totally different state with a totally different bank -- you'd have no idea unless you check your credit report as frequently as possible. Without incurring any fees, you can do this every four months, rotating through each reporting bureau every year. If you know that your spending is under control, and you have few fears about ruining your own credit, it only makes sense to check your credit this way: three times a year, every four months.
However, if you are working to improve your credit, or cannot figure out why some lenders believe you have bad credit, it might be worthwhile to look at two or three reports all at once. You can compare your Equifax to your Experian to your TransUnion if you get all three at once. After all, one of them might be terribly wrong about something, and this is a good way to fix that.
One question about credit report checks that warrants clarification is: does this have any negative effect on my credit score? This stems from a minor misunderstanding about what affects your credit score. It's true that, if you apply for multiple credit products at once, your potential lenders will do a "hard" pull on your credit report, and this can lower your score: it looks like you're desperate for money, and not all that likely to pay a loan back because of this. However, looking at your own credit score is a "soft" pull, and it won't negatively affect your credit in the same way, even if you do three at once.