Nearly every successful person will tell you that goals are a necessity when it comes to achieving success. In the writing industry, I believe goals play an integral part of the process. I encourage all writers to set a specific goal when it comes to how many words you would like to write each day. Write down your goal, post it where you can see the number frequently, and work diligently to meet your goal every day.
What should your number be? Well, that depends on your schedule and your commitment to writing. My word count goal is typically 1.5k words per day. This number changes from time to time, however, depending on my priorities. Sometimes when I am heavily promoting my other novels I lower the goal. Other times, when I am closing in on my deadline (ie: like now with Novel #3) I increase my target goal. This month it has been more like 3k words per day.
With all that said, I do advise taking off at least a couple days during the week whether you're a full-time or part-time writer. I think it's good to allow your mental muscles a rest every now and then. Ultimately, set a schedule, select your word count, and start writing!