Hello,
Some benevolent companies prefer 30 days of basic wage for every completed year of service. In terms of various judgments of the Supreme Court, anyone who has completed more than 180 days of continuous work is eligible for that year's gratuity.
Please also refer to the relevant section of the Income Tax Act, as the amount of gratuity tax-free is restricted to 10 months' pay or Rs. 3,50,000, whichever is lower.
Regards :)