Usually, a thesis is the first book you will write and all professional writers talk about the importance of setting a daily target. When writing up your thesis, a daily target is critical if you want to avoid a breakdown two months before your defense or having your thesis writing drag on and on. You should do this calculation when you’re in the “just-writing-up” phase and you plan to graduate within the next 18 months.
To calculate how many words a day you need to write back-calculate from your ideal defense date. Depending on your relationship with your supervisor knock off 2-6 months from the time you will have the draft done to when your supervisor will let you defend (the closer you work with your supervisor the fewer months you will need for iterations). Then divide the remaining months in half: half you will need for the first draft and half you will need for the second and third draft. Then calculate how many days are in those months (exclude one day per week for rest). Divide the word limit of your thesis by the number of days and voilà you have a daily target!
My own personal calculation: I wanted to graduate in June of 2013 which meant my ideal defense date was in May of 2013. My “just-writing-up” phase started in January of 2012, that means I had exactly 18 months between the beginning of it and my ideal defense date. We had an extra pre-defense review with the committee before the final defense so I knocked off two months for that and another two months for iterations with my supervisor. That left me with 14 months: 7 months for the first draft and 7 months for the second and third drafts. Our department had a word limit of 80,000 words for a thesis. Leaving one day a week without writing, that meant about 500 words a day. (80,000 words/(7 months * 25 days/month)).
Writing 500 words a day at the beginning was hard for me but by the end of the writing process it wasn’t uncommon that I could write 2,000 words a day. If your timeline is such that you need to rely on being able to write more than 500 words a day, it may be too ambitious.