Advice on this matter varies. In his book On Writing, Stephen King alleges he starts the day writing and does nothing else until he has put 2,000 words to the page. This amounts to doing NaNoWriMo all year long. Even if you had the freedom to follow his example, there are things to consider beyond word count.
A Measured Pace
Typing word after word for as long as we can is not the best approach. We need time to think about what we have already written and digest it. Rushing headlong through a draft is almost certain to create inconsistencies. While some may be mild and easily fixed, others will require significant changes, up to rewriting the entire manuscript. Proceeding at a moderate pace allows us to catch issues early and change course.
The most common advice is to write every day, or at least regularly. Once in a while is not enough. A couple of times a week is not enough. Make the time to write five or six days a week. This regularity is important in several ways. First, it allows us to improve through sustained practice. No one is born a great writer; we get there by sharpening our skills. It also lets us stay in the zone and really think about the story, its pitfalls, and ways to make it better.
Furthermore, writing regularly ensures there will be an end product. I probably do not have to explain this, but without a finished manuscript, without finished stories, there is nothing to publish. Lastly, taking the time to actually pound away at the keys establishes to yourself and to those around you that you are serious about the craft.
What Works for You
We tend to have other responsibilities beside sitting in front of a monitor. We may have family members and cats to care for, or the dreaded day job. As such, our time is limited. Setting a certain number of words as your daily goal might not be your cup of tea, or it might not work with your schedule. I use one, of 800 words if you must know, that I rarely fail to meet. You might prefer to set a period of time aside, or your goal might be to write whenever you can.
In short, the answer to “how much should I write” is: quite a bit, as frequently as possible. Find the method that works best for you, be it a word goal, a slice of time, or any other means of ensuring your productivity, and get to work. Because while it can be fun and frequently rewarding, it is a job, and it requires work.