No rule for the same.
Generally Mfg companies have the rule that if a person completes 6 months or more as on 31st mar, 07 (u can have december if u want) he will be eligible for increment which is paid wef the upcoming month of April. (in this case- Apr, 2007)
We used to pay Variable incentive based of the number of month he has worked in the earlier year. But u have 2 be careful for the ones who joined your company between the earlier revision cut off & current cut off date- as in that u pay for a min of 6 months or max of 18 months
PS - Isnt increment prospective & not retrospective???