I wrote my paper in a gap year after MPhil while waiting for a funding for my PhD. I was working then but I had set aside a certain time --Monday&Friday mornings-- for writing this paper. As it was from my MPhil dissertations, it took me only 2 weeks to prepare and then one morning to make corrections suggested by referees. In any case, if you want something to get done, plan it, dont wait for inspiration (it usually never comes;) . And my suggestion is that wrie a paper that is very closely related to you PhD work, then you will get two things at once.