hi rajat,
ur query is very usual for a student preparing for iit with school, as 1st question dat comes in mind how to mange time b/w school n coachings..here i wanna say dat since nw a days jee papers are mainly focussing on school education ,they are giving priority to school knowledge so it wud be helpful in ur case of preparation ....i also suggest u to give good time in reading ncert books as iits r asking questions directly from school books to give preference to school going kids rather dan coaching guys....so just think school as a part of preparation....
Regarding managing time , i wud suggest dat u must join dose coaching which u can go during weekends only so that u can give time in both ....u can study ur coaching materials during weekdays as in school u don't hav much works to do so during week days u can study ur coaching materials n solve problems...and if sometimes u hav some xtra load in school u can chane ur routineaccordingly.....
Also der is nothing like minimum hrs of study , u must study as much is reqrd or as much u can,according to ur capacity.......
All the best for ur preparation and u can ask ur further queries here n we will be happier to solve ur problems
With regards
askiitian expert pratham