The working habits of writers vary. Some can write 2000 words a day or more, others are content with a page while, for some, it is a case of adding a paragraph or a few words to something you have written.
I once saw a brilliant quote concerning a poet. It went along the lines of: This morning I added a comma. This afternoon I took it out.
Our working habits can also be dependent on other factors and it is often a case of doing what suits us best, including the time available and nature of what we write.
But the key is to write something each and every day.
Not only will the words you write add up, the project you are working on get advanced (and if you start to skip days, enthusiasm and the flow can start to suffer), but you will also be helping your writing skills.
Just as musicians, singers, sports men and women practice every day, writing is no different. Writing is a skill and, as a writer, you do need to write regularly and often, no matter how big or small your output is – even if just adding or taking out a comma!