As a freelancer, you may be paid by the hour, by the project, or even by the word. The rates you set depend on many factors, so there's no single answer that works for everyone.
Your level of experience is a key factor. The more experience you have, the more you can charge for your services. The type of writing you are doing is also a factor. Writing that requires a certain level of expertise or experience--such as technical writing, SEO, medical writing, etc.--tend to command higher rates than less demanding types of writing such as lifestyle blogging. You will also want to consider what rates you're willing to work for. If you're trying to determine a fair rate for a specific gig or project, try browsing some similar jobs or services being offered by other writers.