I have seen many students complain of not having enough time to complete their final assignment which makes things hard for them. I think the students should work on their Assignments before time to avoid these last minute rush.

How many hours should a PhD student study?
What are your views?

Most PhD students are recommended to work from 9am to 5pm, 5 or 6 days per week (so a traditional working week), although from speaking to others at my university, most students work more than this.
I think the main issue with writing up the final thesis is that many PhD students can't simply focus on that, they must also take part in conferences, write papers for publications, teach, mark assignments, prepare for progress reviews etc. It's also not uncommon for students to find issues with their project in the final stages, and therefore have to redo experiments, rewrite whole chapters etc. and that all takes time.

I mostly wrote up as I went along, and the content of conference and journal papers became parts of chapters in my thesis. There was a period I then set aside to focus on the thesis in the final 6 months. Ph.D. students I encountered had quite variable working patterns, do what works for you. The hours I worked varied depending on what I was doing, when I had experiments running I would often be in the lab 7 days a week, other weeks would be less intensive. I advise aiming for around 40 hours a week on average, take breaks when needed and don't compare your work to others.