ICB programmes are made up of a number of qualifications (levels). Each programme can range from six (6) months to three (3) years, depending on which course you register for and if you are going to study full time or part time.
There are no restrictions as to how many subjects you can do at a time. It is entirely your decision. Just make sure that you don't double book your exams. Ie don't write more than one exam on a day.
Each subject in the course should take about 48 hours of study before you write the final exam.
Some students do one subject at a time, others do 4 at a time. You can choose which will best suite you.
There are no time limits on completion, at the moment, but each year the syllabi get updated with new content, to keep the material relevant with changes in the industry and new legislation. Especially with Income Tax and Payroll.