Exam leaves for Module E and F and are given according to ICAP guidelines.Trainee students begin with a leave balance of 130 leaves-all the sick leaves,casual leaves and exam leaves etc have to be within this limit for the whole duration of articleship.Approved overtime credited to leave balance account increases your leave balance.
Anyway,to answer your question,you apply for leaves for Module E about a couple of weeks before exams.AFF gives 55 leaves for Module E's first attempt unless your leave balance is too low.And then the number of leaves for subsequent attempts depend on other factors.
For students inducted in firm as internees because they have one or more CA-Inter papers remaining,you apply for leaves before exams and make sure the staff administrator or your job incharge(if you are posted to an audit client) knows that you will need leaves for exams in the mid of the audit.Partners of firms are the highest authority for approving leaves.For 1 paper,you get about 7-10 leaves usually but it depends on circumstances.
As per rules of icap leaves r 130 bt if u do overtime ( eg PTCL and PPAF) the same hours r additional to these 130 leaves. But i guess its better to inform the person who assign jobs to trainee in advance ( atleast 3 months b4 to be on save side).