It averages at 1.8 minutes per mark, so if it is a 10 mark part of a question then you should expect to be writing for about 18 minutes.
Each point you make usually carries about 2 marks (but look at the marking schemes at the end of past exams). For each point you only need to write one or two lines.
I assume you have a Revision Kit from one of the ACCA approved publishers, so look at what they have written in their answers. However both they (and the examiner in her answers) generally write twice as much as is expected (because they know people learn from the answers).