It should be 1.8 minute per mark. You should divide each separate part of a question according to this time limit. e.g 22 minutes [12*1.8] for part a (12 marks) of Q1. If you allocate the time for whole question you'll spend more on some parts and end up leaving rest of the parts of the same question.
If you think you need more of planning before attempting you can allocate 1.5 minute per mark. For this make sure you have practiced past exams under time limit so that you can write faster in less time.
I would keep 1.3 minutes for every mark, it builds in a contingency in case you spend too much time on a given question. Also it gives you a bit of spare time at the end to go back and check or answer any unanswered questions.
1 mark gets 1.8 minutes. In theory papers, division is like this:
Per 1 Mark, 0.3 Minutes of reading time and 1.5 Minutes of writing time.
Whenever the question(or part) reaches its time limit, one SHOULD move on to next.
1.7 or 1.6 min for every mark.
For a 3 hour-paper it's 1.8 minutes per mark.