If I work about two hours on a document, the word count will increase by 400 words. That means that I can write 50 words, or about two sentences, in about 15 minutes, while at the same time fixing other, previously written sentences. Sometimes I throw away whole sentences, so this 400 words takes that into account also. (Rough notes might be faster--but rougher.)
An article is about 6,000 words, so it could be written in 15 writing sessions @2 hours. Let's say that's three weeks of work, at a pretty hard pace. Two hours of writing is a lot; it is mentally taxing. To do that 5 days a week is hard work.
Since I like to take into account the fact that I might be faster than average, let's double that to six weeks. You want to be realistic about how long something is going to take. At the same time, being realistic means not just giving yourself enough time, but also avoiding the trap of giving yourself infinite time. I like giving myself 2 months for something that might take 3-4 weeks. Then I feel great about how much I get done. Or I get it done in two months and still meet my internal deadline.
You can be an extremely slow writer and still get enough written, since steadiness and regularity are much more important than speed. Endurance itself creates speed, in the sense that the manuscript will grow faster with more regular work.