Increasing your word-count by 100 words would involve sitting down for a few minutes at any point in the day and just fiddling around with your document. That's like 4 or 5 sentences. On the other hand, writing a thousand words would be a very good day that you wouldn't expect to have more than a few times a month. So what you would want to aim for is about 250-750 words a day, depending on whether the words were actual prose or rougher notes. A page of really good prose (250-300) words would be very good for a day's work. That puts you on track to write a book in a year of writing, 365 days. You should be able to put together that "year" in the space of 3-4 years.
Usually you only need a second monograph for full professor, so basically you only need to do this year of work at least once between the time you are promoted to associate and some reasonable time afterwards. Yet this second book can be extraordinarily hard to get done. That shows that it is really more of an existential problem than a practical one. I'm going to have to think about some tips to convert existential problems into pragmatic ones with practical solutions.