No rule for the same.
Generally, manufacturing companies have a rule that if a person completes 6 months or more as of March 31, 2007 (you can consider December if you prefer), they will be eligible for an increment, which is paid with effect from the upcoming month of April (in this case, April 2007).
We used to pay a variable incentive based on the number of months the employee has worked in the previous year. However, you need to be careful with those who joined your company between the previous revision cutoff and the current cutoff date - in such cases, you need to pay for a minimum of 6 months or a maximum of 18 months.
P.S. - Isn't an increment prospective and not retrospective?