You can sit it any year of your degree but remember that the results are not valid for long (2 years only I think) so that is why most choose to sit in their final year.
I reckon it's pretty clear that the preparation time will vary from individual to individual. This will depend on
- Their current scientific knowledge on bio, chem, phys
- Their current essay writing skills
- Their amount of free time (i.e. intensity of current course)
I understand that people have spent from around 2 to 10 months in preparation.