Turnoverlimit of previous year or of current year ?

Regarding submission of return it is said that regd. person having turnover upto 1.5 crore will file return on Quarterly basis.. Now here the limit of 1.5 crore is of which period ? Is this mean turn over of 2016-17 ? If a person was having turnover less than 1.5 crore in 2016-17 BUT likely to cross the limit in 2017-18 i.e. current year , Then he will be required to file return on Quarterly basis OR  monthly basis ?