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 factor: The more experienced a freelancer you are, the higher a rate you can afford to ask for. The type of writing you are doing is also a factor. Types of writing that require a certain level of expertise or experience, such as technical writing, 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 decide what a fair rate is for a specific gig or project, try browsing some similar jobs to see what the usual going rate is.