I know this is a very broad question and probably varies for everyone, but generally how long did it take, or do you think it will take to write up? I'm wondering because I'm just starting PhD now and when I'm doing research I generally write as I go, would anyone recommend this?

Definitely write as you go!  Even if you don't use what you write it has you maintaining and developing your academic writing skills.

Yep, write as you go along. If I was presenting work at a supervision, I'd write it as a report (I'm in the sciences) - e.g. intro, methods, results, discussion. A lot of it will be ditched in the end, but if you copy and paste all these reports into chapters then you have a really good start. Official write up time for me after this has/is going to be 4-5 months.

This question was raised on the forum not so long ago. The time frame depends on your subject and how much research you have put in before your write-up year. Sciences tend to take a shorter time, while arts/humanities/social sciences can drag on (obviously due to the higher word count).

Writing up as you go along is a great idea, but depending on the discipline you may find that you need to rewrite it extensively to work the material into the thesis. This happened to me. My early chapters were fine, but to work them into a large humanities-style thesis/argument I had to totally restructure them. Yes I could reuse a lot of material that I'd written as I went along, but much had to be new, and I couldn't just copy/paste to achieve a good result. I'm actually into my third round of writing now, and expect to hit at least fourth before finishing! On the plus side I started writing very early, so I'm still ahead of schedule. As for how long the writing will take well that will vary too by discipline, and in my case it's taking ages. You also have to factor in enough time to deal adequately with supervisor feedback about thesis chapters. If you're lucky that will require minor changes. If unlucky you could be back to the drawing board again.