dear rahul in my opinion it depends on every person absorbing power.just reading of the subjects is not enough because u are going to give ca final examinations.7 months is enough but it depends on ur time schedule how will u allocate time on each subject and the output of that time.i think u r going to give both groups and u need almost minimum 4 months to cover coaching of that 4 subjects.but 3 months is also enough if u have a good command over the remaining subjects.
but don't worry when u feel pressure u automatically become able to manage the time betwwen each subjects.
best of luck brother
regards
tarun rustagi