file cmp 04 from gst portal.....you will be out from the date of filling if you file it voluntarily...but if you disqualify from composition due to non compliance of any condition or your turnover exceeds 75 lakh, than you have to file within 7 days....and you will be deemed to be out from the composition scheme from the date when you disqualify (not the date of filing of form)...
IMPORTANT - If you opt composition by mistake, than there are talks that government may provide an option to those who opted composition mistakenly, to get out from composition retrospectively from 1st july.....next meeting is on 5th august, decision can be taken in that....but I am not sure I have just heard it from a teacher who regularly conducts gst seminar