First, doctoral degrees are not taught degrees and this makes them qualitatively different than any prior educational experience that you may have. At best you might think of them as a bigger, longer dissertation since most UG and Masters level courses have projects, dissertations or theses as one of the final pieces of assessment. However, in most doctorates, the production of circa 100,000 words is the only piece of assessment. Its more marathon than sprint. One former MBA student who subsequently completed a PhD described the MBA as "like a series of assault course obstacles, plan it out, attack the task, survive the exam or assignment and regroup for the next item." Whereas, the PhD process was described as "trekking across the polar cap, on your own, with no buddies and only occasional moral support from your supervisors, friends, peers to sustain you."
In terms of time commitment, the PhD requires regular, high quality attention if it is to thrive. Cramming in two weeks of solid work does not counterbalance months of neglect. Aim to spend at least 8-10 hours a week, almost every week. Binge studying tends not to work well in PhD study. What should you be spending these hours doing ? The two key tasks are reading and writing. The temptation may be there to read a lot but not quite feel ready to commit things to paper. Most successful PhD candidates look back and feel that they should have started writing earlier. The process of writing, even when its ill-formed, inadequate or patchy is better than waiting for inspiration to strike. Make yourself write. There are specific things that it makes sense to formalise and write such as research questions. And try to write in sentences, at least some of the time. Richard Rumelt, the strategy scholar says ...
If you ... put aside the bullet points and just write three coherent paragraphs about what is changing in an industry and why - the difference is incredible. Having to link your thoughts, give reasons and qualifications makes you a more careful thinker and a better communicator.
(in Strategy's Strategist, McKinsey Quarterly, 2007, No 4, p56)
Given that PhD study is going to take you at least 3 years full time and realistically 4-5 part-time, finding the time required is an issue. Its not just a question of finding 8-10 hours every now and then. Rather, it is worth thinking through the likelihood of finding that kind of time week after week, year after year. If you can't commit to this level of effort, the PhD probably won't work out well. Most people, and especially part-time students, have a life outside the PhD and you'll need to negotiate a workable, long-term set of arrangements around these other commitments be they loved ones, hobbies, career ambitions, etc. Also, given that a PhD takes several years many students experience what might be termed "life-events" during their studies. Some cannot be anticipated and you'll just need to deal with those when they come along. Others however, you could see coming if you took a five year time horizon. Be honest with yourself. Realism is better than idealism at least in relation to this choice.