The question of preparation time is asked for two different students and has two different answers: the student taking the course at school and the external candidate.
The student taking the course at school
The programme spreads over two years and the school sets the calendar. The student's job is to prepare for the exam format on top of that calendar.
The real work here is not learning content but settling the form of the answers through past papers and mark schemes. Started a few months before the exam, that work is sufficient.
The external candidate
Duration is decided by the number of subjects. A few months suffice for a single subject, while six subjects means working a two year syllabus from scratch.
This is the source of the most common planning error: looking at the time for one subject and not multiplying it by six. Studied in parallel the total time lengthens, because the weekly hours are divided.
The binding constraint is registration
However long preparation takes, the centre's registration deadline binds and falls before the official one. For the May series, registration closes in winter.
The calendar is therefore built against the registration date rather than the end of preparation.
Splitting across series
That the IGCSE has more than one series a year makes it possible to split the load. Three subjects in May, three in October.
That produces a markedly better result than sitting six at once, and most students do not know the option exists.
When to start
For a student taking the course at school, exam format work three or four months before the exam. For an external candidate, one or two years ahead depending on the number of subjects.
In both cases the first thing to do is find out the registration date and build the calendar backwards from it.